The problem with "developing psychoses" is that it's unrealistic. Hit men don't become serial killers, serial killers are mostly made in childhood (number one common trait is inconsistent discipline, according to an FBI study - either the discipline was completely random or ridiculously harsh). They may be drawn to that particular career choice due to their own desires, but it doesn't make them suddenly want to start killing more. However, if we assume your character has the general background to develop these things, what would be more likely to trigger them would be a stressor. Generally speaking, any non-genetic mental disorder is almost always preceded by a period of high stress. So for example, if your character is a thief, he might become a kleptomaniac after the death of a loved one, or after being nearly beaten to death by cops or something. A hit man with a serial killer background might suddenly hit on his fantasy (which was hidden from the player) and start suffering from stress and desire to do... something to that NPC he just saw. Fulfilling the fantasy would remove the stress, which would be very difficult to calm otherwise.
I think the problem with this simulator being a general crime simulator trying to pick up fame in the wake of the SKRoguelike is that serial killers are not normal killers. They kill for a reason nobody can understand, they do strange things to satisfy an internal urge that even the killers themselves don't know the source of, and they are rarely interested in what normal people would be. They can't be reasoned with like a normal human, the link between them and their victims has more to do with their internal fantasy and random chance, and they are often unable to control their actions unlike a regular criminal who could stop, albeit with some danger. Treating them like a normal criminal works for a game, but if, as Cthulhu says, you want realism they have to be treated as a third classification of human being - law abiding, criminals, and serial killers. Trying to model them with the same motivations as normal criminals won't get you the depth of immersion you're hoping for.