Todays' term? Psychopath.
DUDES! People use this wrong all the time. The confusion here is kind of understandable. Psychopath sounds a lot like psychotic/psychosis. They are different. If someone's acting all crazy-like and seems to be a few guns short of an arsenal (note: please don't give psychotic people guns) then they are psychotic. It's a sort of generic, blanket term for someone who's nutso. Like how shoe is the generic, blanket term for something you stick on your feet before you leave the house.
Psychopathology is totes different, y'all. It's specific. Calling someone a psychopath when they're being psychotic is like calling all shoes lace-up ballet flats with sparkly trim or something. A psychopath is a person who, essentially, has no empathy and no conscience. They choose their behaviour based solely on the reward it will bring them and don't form any of those pesky human connections that make the rest of us do chump-like things that help others and not ourselves. They are scary as all h-e-double-hockey-sticks because there is NO WAY to recognize them before they are murdering you, stealing your money, sleeping with your wife, or whatever else they decide to do.
For reals. You know how people are always like "oh, he seemed like such a nice person!" when they find out their neighbour was storing body parts in their freezer? It's because psychopaths, while being devoid of real human connection, are so effing good at faking it that people naturally like them. They are super outgoing, friendly, smart, and interesting people. So much so that they are often the kind of person that people love to be around, a person that people will naturally trust. They are not all creepy, sitting off to the corner with dark eyes. They are smiling and laughing with you while they wonder if your home is soundproof enough for murder. Or whatever.
This is why, if you ever meet someone that seems really nice and that you naturally like right off the bat, run the other way. Only make friends with people who seem really shifty, because while they might also be wondering if your home is soundproof enough for murder, at least you won't have that sense of foolish betrayal when they try to do it. You'll be like "yes, I totally expected you to try to kill me because you were acting like you were a killer, this makes perfect sense" instead of "OH M G WHAT ARE YOU DOING I TRUSTED YOU!!!" It would make being murdered really suck.