post an insane anime character but still cute

Diabolik Lovers.

Profesor Franken Stein from Soul Eater

Though he gets consumed and controled by evil madness by a demon god, without it, he's still pretty insane! But he can also be charming and funny all at the same time XD He's insanely and adorably mad :3

Yuno Gasai from Mirai Nikki!

Junko Enoshima

Tamama! His hobbies include:
-Doing anything and I mean ANYTHING to get his beloved Sarge's attention
-Making That Woman suffer
-Killing flies with his crazy rage breath
Oh yes, and he loves pocky. All flavors, he says.

Kamui from Gintama

Liang Qi from Canaan

Rena Ryuugu from Higurashi: When They Cry. She's so adorable when she's not hacking someone up with a cleaver~

Alois Trancy from Black Butler

Amaimon.
He's cute as fuck.
I wouldn't exactly say he's INSANE, buuut...he is very sadistic and gets bored extremely easily, and the only way he can cure his boredom is by fighting someone strong enough that he won't kill almost instantly. (or not so strong that they'd kill him)
Which is why he's so obsessed with Rin and wanting to make him suffer. He's the only working toy he has and he barely gets to play with it. (probably in Amaimon's eyes. XD)

Nena Trinity from Gundam 00
(in)famous for bombarding a wedding.