Complete Monster/Sonic the Hedgehog


 * Shadow the Hedgehog's Black Doom counts. Doom's invasion actions seem similar to terrorism tactics. His plan is also gruesome: have his armies get strong by devouring every sentient being on the planet and terraform it to better suit his kind. While he does pet Shadow, this is more because he needs Shadow on his side than genuine affection. And it's also heavily implied that the Black Arms are being mind-controlled, if not just near-mindless anyway. Meaning that any Pet the Dog moment with them (and, by extension, everything he does) is purely self-serving. And unlike most villians in this series, he doesn't even have a dry sense of humor. Something even Mephiles, who's basically the DEVIL, has.
 * Ix also qualifies. We're talking about a guy who rules his people with an iron fist and developed unstoppable weapons and robotic warriors in a bid for global conquest - and he would have gotten away with it had the Argus Event not occured (said event shunted his entire civilization into a parallel dimension whose flow of time isn't parallel with ours and is also a Tailor-Made Prison to other races of similar renown). However long his people have resided in the Twilight Cage wasn't long enough for him to repent, it seems, as once the Nocturnus have developed interdimensional warp devices, he's at it again, and his response to Procurator Shade questioning his motives is to blow her off Angel Island, which gives her suitable enough motivation to change sides. The guy is known to execute dissenters on a whim to boot - Shade, in one conversation, states that she is siding with Sonic and company with the full expectation that Ix will do everything in his power to have her killed. Now tell me the guy isn't a Complete Monster.
 * Mephiles certainly qualifies. He wanted to try and essentially eliminate Time itself, as well as the fact that he killed Sonic the Hedgehog for little reason than to make Elise cry so he could merge with Iblis and thus destroy time itself. Although Sonic got better, it's still quite the Moral Event Horizon. Its also implied that he was part of the reason why Shadow ended up sealed away by humanity as well as an unwilling Omega carrying out the deed (he had some resemblance to Shadow thanks to absorbing his shadow, so its not unreasonable that he would have done deeds that the populace will blame Shadow with), and he manipulated Silver into attempting to kill Sonic with the implied intention of destroying the past. If that's not enough, he also enjoyed the carnage he inflicted. He currently ranks highest in regards to being the most evil character in the series not only because of his motives simply being to wreck and destroy anything and everything he can for no reason other than For the Evulz at best (even Black Doom at least desired to rule over the Galaxy), but also at how frighteningly close he came to succeeding in his goal to destroy everything, heavily damaging the time-space continnum.
 * Eggman Nega would certainly qualify; he was destroying the natural balance of the world and doesn't care that either side would be destroyed. He's basically what happens when Eggman cuts the crap and gets down to business. The fact that Nega is his descendant doesn't help; he's willing to essentially destroy everything via paradox. The one time he teamed up with the normal Eggman, even Eggman was horrified by how evil this guy was.