An insane anime character?

Ladd Russo From Baccano

Mephisto doesn't exactly seem like the most mentally healthy.
An oddity among demons and insane among humans.
Oh, how I love Mepphy~ <3

Shou Tucker from FMA, who first turned his wife into a pained monster just for a research grant, and later later did the same to his dog and child.
Don't you find this sick? XC

Karma Akabane from Assassination Classroom is one insane and sadistic kid.

Yuri Petrov from Tiger and Bunny. Anyone who stands on a rooftop in the middle of the night in a spandex suit and rants about the voice of Thanatos needs help.

Donquixote Doflamingo (One Piece)

Harime Nui from Kill la Kill. She gives me chills down my spine just by thinking about her.

The majority of the cast of Higurashi no Naku Koro ni.
Below: Ryuuguu Rena.