I've seen these kinds of things a lot- from killers for no reason to "emotionless" characters and such. To be honest, it doesn't bother me, but when "Killers" are "killers" for no apprent reason or backstory is sometimes annoying even though I don't pay much attention to it.
My only character classified to be "insane" is Viola. She's schizophrenic and it led to cannibalism- but at least she was constructed as a character.