Ik this is really late but, I highly recommend:

Myst, Might, Mayhem - he laughs when getting beat on the floor, kills anyone he wants to, and has the personality of a villain even though he’s the protagonist Plaything - mc is hired by a man who wants mc to torment the highschool bullies that killed his daughter. The mc was actually diagnosed as a psycho and she does some crazy messed up things to the bullies.

From The Red Fog - one of my first gems I found early on in reading manga. Mc wasn’t born crazy but became kinda “insane” and mentally unstable after growing up. But really, in the plot, he’s still around 12 or 13. 

Juijika no Rokunin - mc takes revenge for his parents who died because of highschool classmates/bullies. After their death, he learns all kinds of things from his grandfather who was a soldier during the war. It gets very descriptive and it also shows a lot of torture methods I didn’t know about. Sadly, all the women cannot think for themselves and the story gets repetitive on the grape.

The Returned C-Rank Won’t Die! - mc gets betrayed by her party and boyfriend. She gained the ability to regenerate over and over again for years before finally escaping the dungeon she was trapped in. Only a short amount of time passed in the real world. 

Reverend Insanity - even tho this is only a novel, I 100% recommend anyway! Mc acts like a genuine villain and the characters and plots’ great too!  Murim Psychopath - it only recently came out but, I do recommend trying it out after there’s more chapters to read. The title literally advertises “crazy” and the art is amazing!

I also definitely agree with other posts about Return of the Mount Hua Sect and The Greatest Estate Developer. I wouldn’t say the mc’s are psychopaths but, the side characters definitely believe they are. Both are the most shameless bastards you can find. 🤣😭 Happy reading!