Yaoi or straight

http://www.mangago.me/read-manga/hanazono_no_kioku/ he's not really a psycho though he's got his moments

http://www.mangago.me/read-manga/yume_touka_end_roll/ (i don't really remember, but i think seme has mental disease
http://www.mangago.me/read-manga/jikan_akima_jinbutsu/ serial killer falls for another person
http://www.mangago.me/read-manga/yami_bl/ last ch
http://www.mangago.me/read-manga/nii_chan_harada/ (in love with pedophile)
http://www.mangago.me/read-manga/to_the_sea/
http://www.mangago.me/read-manga/skin_asada_nemui/ (in love with pedophile)
http://www.mangago.me/read-manga/konoyo_no_hate_made_i_love_you/ this one is cute, falls for his stalker xD

maybe not exactly what you are looking for but it is similar

http://www.mangago.me/home/mangalist/57857/
Check out the mangas with - twisted,psychotic characters & murder mentioned in their reviews. But I specially recommend trying "Boys Next Door" by Yuki - http://www.mangago.me/read-manga/boys_next_door/

Omg all of ur suggestions were all great loved every single one arigatou (๑⁼◡⁼๑)

Yay, I'm glad :)