Role in the series
Eggman is a short-tempered, raucous, pompous, cruel evil genius who is constantly planning to take over the world. His primary motivation throughout most games is his psychotic belief of self-entitlement. Arrogant and narcissistic, he constructs countless technological wonders in his own image to fuel his massive ego, usually reinforced by his goal of finally constructing the Eggman Empire, and doesn't care where it starts.
Although Eggman is a villain, he helps Sonic when the world is at stake; though the reason is always because he knows there would be nothing left for him to conquer if the planet was destroyed. In some of his incarnations, he actually displays a begrudging respect of his arch-enemy, Sonic.
Despite his intelligence, Eggman can be very immature, throwing temper tantrums when Sonic manages to wreck into his plans. An example of his immaturity is in the ending of Sonic Unleashed, when Orbot begins to insult him over his constant failures and calls him a loser; Eggman screams "Oh, shut up!!", attempts to kick him only to comically fail, and then chases him through the desert.
Eggman seems to overlook crucial details in his plans, such as in the opening sequence of Sonic's story in Sonic the Hedgehog (2006) when he orders his robots to fire at Sonic while he is escaping with Princess Elise, failing to realize that they could possibly injure or kill Elise if they land a direct hit on Sonic and overlooking Dark Gaia's immature release in Sonic Unleashed. He also was shown in Sonic Colors to be somewhat sarcastic, as evidenced when questioned by Orbot and Cubot as to whether he wanted the remaining aliens, to which he told them in a flippant manner that he actually wanted a cheeseburger and a shake (although Cubot ended up misinterpreting it as a genuine request).
While Eggman's motives for conquering the planet and attempting to wipe out entire populaces are never made clear, his grandfather's back-story implicates the strongest motive. Gerald Robotnik was one of the most brilliant and gifted scientist the world has ever known and was immensely adored and respected by his grandson, who dedicated his life to becoming a great scientist like him. However, despite Gerald having nothing but the purest intentions to use his genius to better humanity and his family (specifically his granddaughter, Maria), he was brutally manipulated by the government into producing research that they hoped to twist and exploit towards their own ends to create weapons of mass destruction and even discover the secrets of immortality. When they deemed Gerald's research too dangerous, they viciously murdered all his colleagues, arrested him, and killed his twelve year old granddaughter, who was also a cousin to Eggman, simply to cover the truth about the research and experiments. Forced to continue his experiments under direct military control within a prison facility, Gerald completely lost everything to live for and went insane, eventually being executed by the military under government orders to keep their crimes quiet once they deemed his research a failure.
With his idol, a man with not a single bad bone in his body, condemned to such horror by his own government, Eggman saw humanity as power hungry mongrels who destroyed the lives of much of his family. No longer seeing the logic of using his similar genius to help such a society and world, Eggman then sought to become a great scientist for the purposes of avenging his grandfather by conquering the planet. However, Eggman was eventually diluted by his own lust for power as he began to make plans to conquer areas outside of his own world and became obsessed with killing Sonic and his friends in the process and obtaining revenge for their constant interference in his plans. Whatever his initial motives for his plans may have been, it is clear that Eggman is now primarily concerned about obtaining power and dominance for himself, with the legacy of his grandfather currently only a fleeting motive at best, with his violent tendencies and insanity mounting as the series continues.
To this end, Eggman has never shown a willingness to do good or help others without it serving his own ends in some shape or form. While displaying a comical disposition later on in the series, he is quite clearly a cruel minded and brutal man who will use any opportunity presented to him to fuel his self-serving desires, even if that means exploiting worldwide catastrophes to obtain power and dominance. From abducting and transforming animals into robot armies, to leading large scale attacks on a terrorist level, Eggman's numerous crimes and exploits continuously transition into more destructive ramifications. He never acknowledges the results of his actions such as the pollution from his fortresses and strongholds that poison the surrounding environments or the ruins that are left from his own attacks on various cities and lands. His exploits have led to numerous tragic events, such as Chaos flooding and utterly destroying all of Station Square and Dark Gaia nearly consuming the whole planet, as well as leaving numerous areas, lands, and even planets in ruins and threatened the welfare of time, space, and the universe itself.
However, despite his desire for world domination, Eggman does have a human side, and does not appreciate senseless destruction, tormenting and killing from other individuals and forces who are beyond his control. InShadow the Hedgehog, he is shown to be disgusted and shocked at the thought that his own grandfather would sacrifice his own people for the sake of research and aid the Black Arms. This demonstrates that even if Eggman does not mind causing chaos and destruction, he ultimately knows that there is a line that others should not cross and was afraid his grandfather did so through betraying humanity and aiding a species that wanted to eliminate the human race. This shows that the doctor has some form of loyalty to the human race and could not fathom the idea of betraying his own species to that of a vicious and demonic alien empire, despite some of their actions mirroring his own in terms of the scope of their plans and the destructive ramifications of their actions.
While this seems contradictory towards Eggman's previous exploits of joining forces with other creatures bent on destruction, it seems that he still wishes to paint his grandfather in a positive light as he was his hero in his childhood. This more humane side to the doctor is shown in full force at the end of Sonic Adventure 2, where he has an honest and genuine conversation with Tails about how much he adored his grandfather, questioning the actions of his fallen idol and wondering if he intended to destroy him as well. Additionally, in Sonic Lost World, he unhesitatingly jumped in to save Tails from being attacked by a Crabmeat-controlled Cubot, and when questioned by Sonic over having rescued one of his worst enemies, Eggman admits that he's "a complicated guy."
Despite this, Eggman seems to have no problem in committing senseless killing and destruction as long as he is the one in control of the forces doing so. It would seem that the doctor is so bent on authority, that he refuses to let other forces destroy his planet unless they are under his complete control. This is shown when Chaos betrays Eggman and floods all of Station Square, an action that Eggman had planned Chaos to do from the very beginning regardless of the beast's betrayal against him, but refused to allow his rampage to continue as Chaos was no longer under his control. Additionally, in Sonic Adventure, he launched a missile deep into the heart of a heavily populated Station Square to utterly destroy it, even if that meant killing himself (even going as far as to state 'I'll take the whole city with me'), as a twisted form of revenge and victory over Sonic after his plans to use Chaos to dominate the planet failed. In Sonic Adventure 2, his desire for destruction appears at its height as he laughs insanely at the Eclipse Cannon's impending firing at the planet, which would have had devastating effects to billions of lives, until his grandfather's true operations were activated. He also admits that had he had the chance to use the space colony ARK to collide with Earth, he would've done so a long time ago, showing admiration towards his grandfather's mad projects, and only decides to prevent them from coming to fruition in order to save his own life as escape was impossible with the colony on its collision course.
In Sonic Unleashed, there is a possibility, realistically speaking, that he murdered millions when reawakening Dark Gaia by splitting the planet itself apart, though the effects of the quake in relation to the earth's populace is never touched upon. In Sonic Lost World, Eggman contemplates destroying one of his own doomsday machines to ultimately destroy the Deadly Six, even if it means taking many innocent lives with them, much to Sonic's disgust. According to his PA announcement in Sonic Colors, his park can be considered a lethal weapon in itself as most of the rides lead to horrifying and agonizing deaths and circumstances should anyone ride them. Lastly, in the beginning of Sonic the Hedgehog (2006), he orders his robots to launch numerous missiles at Sonic with no regard for any that may be caught in the crossfire.
Regardless of how many lives may or may not have been lost to Eggman's various schemes, the evil scientist clearly demonstrates a complete disregard for all life, even his own on occasions, as well his own creations, and will go to any means necessary as long as his goals of destroying Sonic and his friends, conquering the planet, and establishing his empire are met with full success. With the means for Eggman's plans to succeed often resulting in massive amounts of chaos and destruction, the success of his schemes would also result in a world slowly dying away from Eggman's ruthless and tyrannical rule. The various, dark futures within Sonic CDprovide a glimpse of the horror that would be a world under Eggman's control; twisted steel, pollution, and ruin, to the point that even Eggman's own creations are in pitiable states, illustrating that once Eggman has seized control of the world, he will become a ruthless tyrant who will exploit all of the planet's resources to the point that they are polluted or depleted as even his own creations suffer. This could imply that once Eggman has conquered the world, there will be little need of his robotic army as he primarily uses them to conquer territory and will be seen as expendable to him and left to rot. This would further be indicative towards how Eggman has previously treated his robots if they failed him, ordering torturous redesigns on his various E-Series robots if they did not complete their missions and even abandoned Orbot and Cubot in the vastness of space without a second's hesitation, though they would later rejoin the doctor sometime later despite this. Also, in Sonic Lost World, Eggman is contents with conquering what is left of the world after the Deadly Six's actions, supporting that Eggman does not care what condition the world is in as long as he is the one ruling it.
Perhaps the most frustrating and deadly aspect about Eggman is his tendency to always escape at the last minute, even as all his creations and machines crumble and blow up around him, ensuring that he will always be back again with another scheme to endanger the planet and beyond. No matter how close Sonic appears to be finally catching the villain and putting him out of commission, the doctor always has an escape plan, which ironically enough, are the only plans he can formulate that Sonic has been known to fail in foiling, making it nearly impossible for both individuals to defeat the other for good, making their battle a never ending one.
One of Eggman's more admirable, if not dangerous, traits is his courage throughout life threatening situations. Despite Eggman's continuous use of machinery to both protect himself and fight his opponents, Eggman shows considerable bravery throughout the entirety of the series and very rarely shows fear, regardless of who or what he is facing. Even in the wake of the Black Arms invasion, Eggman remains very composed when facing the demonic and destructive race and even enters their base of operations, the Black Comet, himself without showing any fear. His persistence to control other creatures bent on mass destruction and approach them to ultimately tame them, including Chaos and Dark Gaia, also illustrate his apparent lack of fear.
Eggman, while sending out his army to do most of his dirty work, has also been known to lead missions personally with himself in the field for both amoral and noble deeds, though with self serving interests always at the forefront of his mind. His apparent lack of fear during these missions can be owed partially to the fact that Eggman has previously shown a disregard for even his own life on several occasions (as in the case of his attempted suicide bombing of Station Square with a missile), as long as his plans come to full fruition. In Sonic Adventure 2, Eggman tells Shadow that he might not survive his battle with Sonic in the space colony ARK and instructs him to finish their plans to use the Eclipse Cannon to dominate the planet and possibly even use the weapon to destroy large portions of it. While the villain has been known to panic when his plans spiral out of control, especially when victory previously seemed assured, he is always able to compose himself long enough to either finish the mission at hand or escape at the last minute.
Ultimately, while not openly violent and ruthless, Eggman is still a highly brutal and amoral villain with an insatiable lust for power, dominance, and, since meeting Sonic, revenge. While still harboring a more humane side and displaying comical undertones, Eggman's ruthless and violent nature can still be seen throughout the series. His uncompromising persistence shared with his appetite for authority and control fuel the conflicts throughout the entire series and guarantee that Sonic will always be needed to stop him.