It's always amazing to think about the amount of impulse control you go through daily. I like to think I'm a pretty good human being, but I have violent thoughts, lustful thoughts, and even some sadistic thoughts on occasion. It has become easy for me to shrug these off each and every day, but it's painfully obvious that there are plenty of humans with far less impulse control. In fact a great deal of society basically exists in an animal state where actual control of their lives is based on what happens to them rather than their internal choices.
You can look at any human being's life and pick out horrible things they have done. It's just human nature that we try to overcome through education. That's the easy explanation. You can look to religion if you want an more in-depth or outlandish explanation.