For me, I'd have to say probably Gasai Yuno from Mirai Nikki, who I doubt needs much of an introduction, if at all:
Or maybe Professor Salva from Sankarea (the manga only, though):
I would like to make it clear that that's a
guy. So... yeah. That's kinda creepy right off the bat. Well, he also tried to kill his daughter with a flock of cybernetic attack-owls and has(had) an extreme obsession with creating zombie lolis and naming them all "Rosalie". So there's that, too.