I like antagonists that are consumed by madness and meet their end because of that madness.
They go deeper and deeper into a rabbit hole. So deep that they can't get out -- they've done some messed up stuff, and while they have their reasons, they're not enough to ever excuse what the antagonist has done. Hell, their redemption might only be met through death.
Even better is the horrible realization that all they did was for naught. It's chilling because you can often pinpoint that moment: maybe it's their facial expression or maybe it's the visuals, but it's clear they messed up, and what they really wanted wasn't something they needed.
And then they break down.
-Chef's kiss- It's beautiful.
Edit: Bonus points if they've managed to drag the protagonist into the rabbit hole. Whether or not they make it out is up to chance, but it's a beautiful hot mess.
Triple bonus points if there's, like, some oddly-enough sensual seduction to the dark side during all of this.