You're a product of your own enviorment. If you grow up in a neighborhood that's riddled with violence and poverty, then thats all you know. The tempation to rob, steal, and sell drugs is too great knowing that you'll make more money than anyone you know working a legitimate job.
That's only speaking for people in ghettos and lower class areas of America. Everyday people that don't need to commit crimes? I guess for the rush and excitment. My friend is a clepto and will always steal a candybar or mach3 or something from a supermarket cause in the middle class neighborhood we live in, he knows he won't get caught. and if he did he could talk his way out of it. He's not too well off so he steals when he can. I guess then it's a mix of nessessity and adrenaline. I would to, but I haven't stolen since I got caught in like 5th grade lol.
Or are you talking about those serial killers? That's a whole different ballgame altogether.