i'm still largely unfinished and has just began with this part of my list but it's basically the type of the characters you are looking for (the mangas in this page ) http://www.mangago.me/home/mangalist/69274/?filter=&page=5
http://www.mangago.me/home/mangalist/57857/
I have mentioned the ones that have psychotic twisted characters in them, in my review notes. But I would recommend you picking up ANYTHING by Harada sensei. She'e the QUEEN when it comes to portraying psycho/sick characters. For starters try this one -
http://www.mangago.me/read-manga/gintama_dj_mienai_kusari/
Can anyone please recommend me any manga that the characters are 'sick' to the core?? Or just a psycho??
Thanks