Sigma...Ohhh boy, you don't want to mess with that guy.
To me, I think Sigma was an evil demon from his inside of the head, but on the outside, he "pretended" to be a loyal gentleman in the Day of Sigma OVA. But due to the virus that was inside of him after he fight Zero and broke his crystal, his mind went completely insane, but it came in his mind pretty slow than the current Maverick Virus that other reploids/animaliods had. During that OVA, he begins to get pretty annoyed about World Peace and he wanted to create his own world to turn it into an totalitarian world. However, this causes him to become evil, and he was permanently infected, due to his broken circuit. When his main body is destroyed, his own virus has been created and spreads into the next body, causing to infect computers and military weapons, which caused the Hunter Unit's AI brains to be infected as well. THIS, however can happened if the reploid implants Sigma's mind into the victim. It can also infect the human being because it can successfully tap into the human’s nerves and HE controls them (according to the Rockman X5 Manga that I read).
Anyway, I hate Sigma, and he is a [Top Spin] because he tried to harm a human nature and tried to ruined the future for us. He maybe cool for some of you, but he is REALLY dark nature.