From what I've read they think it's genetic.
No more than anything else. Anyone can claim something is genetic, but the environment plays a big role in behavior. A person may have a gene that makes them a more aggressive person however not all aggressive individuals go on to commit crimes or put forth psychopathic behavior. Just like not all individuals who grow up in a horribly broken home with bad luck in everything are going to be delinquents.

Psychopathy is not common. A person who is just a downright mean spirited person is not a psychopath. A person who steals money, laughs at other's misfortunes, or shows no guilt for being rude or crimes committed is not a psychopath they are troubled and may have some social disorders but they are not psychopaths. A psychopath is the one that gets passed a lie detector test, is incapable of true love, gets through life coasting on others, and smirks at their success in puppeting everyone else. Their charm will ruin all in their path. But the Joe Blow who is a natural D-bag just causing grief for everyone else is not a psychopath and that is what makes things so interesting. Everyone wants to put a label to someone's behavior when they act out of the norm, naturally something extreme like psychopath is brought up because it is a harsh description, but there are so many actually common personality disorders around that the true psychopaths are rarely actually found.

One analogy I read once that I kinda agree with is that if you really want to see a psychopath in action just watch some politics. See how they lie to get their way, they are masters of manipulation, extremely grandiose, seem to lack empathy for what is most needed, impulsive, greedy, fail to take responsibility, they have many behaviors which one could classify a politician as a quick example of psychopathy.