Snively Robotnik was born as Colin Kintobor Jr; and betrayed his family to help his uncle Julian (better known as Ivo Robotnik) take over Mobius. He hates Sonic as much as his uncle, due to the fact that Sonic took away Snively's hair by running by him too fast, knocking it off. Snively was originally a faithful sidekick to his uncle, and tried to destroy the hedgehog many times to earn his uncle's respect. He failed every time, and with every failure, Robotnik grew madder.
Snively eventually grew tired of his uncle, and at the conclusion of the "Endgame" saga, he intentionally made his plan "backfire" and kill him. Snively went into hiding soon after.
The New Robotnik
When Robo-Robotnik of the other dimension rose from destroying his dimension's Sonic, Snively began working under him. He kept many secrets from his new uncle, and planned to betray him as he did the original. After many failed attempts, he eventually took over the Eggman Empire.
A Second Betrayal
Soon after the war against Moebius, Sonic, the Freedom Fighters, and the Chaotix launched an assault on the Eggdome, Eggman's base. Eggman, who had almost lost his mind, made it easy for Sonic to come into the Eggdome without backup. Sonic went in, and was surprised at no defenses. He found Eggman, and destroyed his newest robot, the Egg Tarantula.
This was Eggman's last stand. He ripped off his mustache and went completely insane. Snively then tricked the Mobians into thinking they won, while really awaiting his newfound love, Regina Ferrum, the Iron Queen.
The Iron Dominion took over the Eggman Empire, but when their rule ended, so did the affair. Snively let out his uncle, with regained sanity, and though he promised to his uncle that he was over Regina, Snively is to this day planning in the shadows...
Sonic the Hedhog (1993)
In the TV series he started out as a loyal servant to his own Uncle. However by the start of Season 2 he showed hints of being irratated towards his uncle and would often mock him when he's not listening. By the end of Season 2 Snivelly survived the destruction of Robotropolis thanks to the power stones and proclaims to be the new ruler of the city now assuming his uncle had bitten the dust. Upon cancellation of the show, a third season never got made and would of depicted Snivelly as the new villian of the freedom fighters.