honestly depend on your definition of psycho, if we go by the definition I can t remember on top of my head any one that are worse reading, I mean Redo of the healer probably fall into it but I can't be arsed to hyperlink it
if you just want a protagonist that do not care to kill if they accomplish their objective you can find a lot of good one by just using advanced search and using
Seinei + gore (obviously most are not what you are searching but you will be able to find more than what I can pull from the top of my head)
But here some recommendation of borderline psychopathic MC
BLAME! ... I mean the MC name is Killy and he barely talk or give a shit about what you have to say, he will do what it take to find his goal
CHAINSAW MAN While denji is not a psycho and he still care about the other, he have not given a single fuck about killing people he do not care for, nor do he really care for justice itself
Berserk some arc are probably right up your alley
DoroheDoro moral does not even exist in this universe, it is gross, unkempt, but you will still care for it's character and that what most psycho random manga lack... but well again the character are not psycho in themself, it just that nodoby care for life
Heroes actual psychopath by the author of
Goodnight Punpun
Drug it! it is a one shot so easy to read
The Voice of a Mermaid same
overall I do not think my recommendation are what you are looking for but if you want to try them out please yourself
you could also find a lot of psycho in fighting manga, but often they have not real main characters so maybe not what you are trying to find
