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.
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.
Erazor Djinn from Sonic and the Secret Rings. He blackmailed Sonic with an inevitably fatal curse if he didn't gather the World Rings for him whilst having no intention whatsoever of removing the curse if Sonic did fulfill the terms, intending to use him as a ritual sacrifice to gain the true power of said rings. He tried to murder Sonic, but only succeeded in killing Shahra in his place, and even though Shahra's killing was unintended, he still had a Kick the Dog moment in mocking her comparatively weak magical power as a genie. On top of that, he even had the audacity to manipulate Shahra into stopping Sonic from dealing with him. She doesn't, however. He was defiant right to the very end, even whilst Sonic was throwing him in molten metal.
