Nui Harime from Kill la Kill. Such an obnoxious character who was way too cheerful and bubbly when committing treacherous acts of murder or talking s**t about others.
Discord from My Little Pony: Friendship Is Magic. Now I don't entirely hate the character. He was an entertaining villain and still has some enjoyable moments after his reformation. The biggest problem that I had with him was when he betrayed the entire pony race to Lord Tirek in the season 4 finale, bringing Fluttershy to tears which is unforgivable in my mind. For quite a while there was something else about his betrayal that bothered me, but I couldn't quite place what it was. Then a YouTube Brony analyst by the name of Silver Quill brought up a strong point that gave me added clarity. He mentioned that in the episode Keep Calm And Flutter On, when Fluttershy denied her friendship with him when he took advantage of it, that it forced him to make a decision. That decision being either absolute freedom or the first real connection he'd ever had, which he ultimately chose the latter. So when Tirek offered him absolute freedom if he helped him drain ponies of their magic, it didn't make sense for him to choose that after already rejecting it in the past. The one thing that bothered me the most about his betrayal was that after Tirek was defeated everypony was far too forgiving of Discord. What he did was a really as*hole move on his part and yet they treat it like he'd just spilled milk on their tablecloth or something. If it were up to me, he would've needed to work hard to gain everypony's trust back. Again, I don't hate him, but he's got a good ways to go before he can get my trust back.
But by far there is no character from MLP that I hate more than Sludge the dragon from the episode Father Knows Beast. This character (if you could even call him that) lies about being Spike's dad and takes advantage of his trust just to get a taste of the good life. He's manipulative and has no concern for anyone's well-being but his own. Spike even lashed out Twilight when she expressed concern of Sludge taking advantage of him, literally breaking her heart in the process, after she's practically raised him for the majority of his life. What's even more painful was that I predicted that Sludge wasn't really Spike's dad and that he'd try to take advantage of him, but I still watched the episode in the blind hope that there'd be some sort of redeeming quality about him. This wasn't the case though and I didn't even bother watching the rest of the episode when Sludge revealed his true colors. To this day, that is the only episode of the show that I never bothered to finish. Sludge is so absolutely despicable that calling him the scum of the Earth would be far too strong of an insult... for the scum! He's a complete sh***y douche f**k that I'm sure will never be topped for however longer FIM is around for.