It never ceases to amaze me that as a species, humans, supposedly evolved, don't recognize stupidity when acting in evil ways.
Whether lying, cheating, stealing, or just being mean, manipulative and/or selfish, these things are observable by all other humans, and "the truth will out", as the old saying goes. At some point, actions catch up to the actor. Even if the actor is oblivious to their own stupidity, fools themself into believing that they're Teflon, it will, in the course of living with others, become painfully obvious.
So why do people do this? Why do people do rotten things? Not believing in an afterlife or karma is no excuse; you still have to get through this one. It's got to be joyless and lonely to choose that stupidity.