Guilt.

If there was a human emotion that I hated more than any other, it would have to be guilt. Guilt is the jerk that stabs you in the back after Hatred, Tragedy or Rage force you to hurt someone. It's what stops a lot of good people from just dropping the Superhero act and running around town laughing maniacally and blowing things up. I, personally, am not a very big fan of guilt. Guilt holds me back from kicking the asshole who pissed me off, from screaming at the teacher who was being a neanderthal in terms of the judicial system, from smashing the stupid bully who stole my lunch money's head in the bricks and a hell of a lot of other things.
But we've got to remember, without guilt, there's not a lot of right or wrong. Guilt is a very human emotion, we often feel it as some sort of anxiety or pain whilst watching an evil deed or after having done one. Guilt is often interpreted as a sign of goodness in a person. Not feeling guilt, means you don't feel sorry. You don't feel bad for having hurt someone. You don't feel bad for sinking to their level if you just succeeded on your revenge quest. To put it in simplistic but succinct tones, it means you're evil.
Sometimes, being evil seems easier. You can do whatever you want whenever you want and you don't feel one teensy bit worse. In movies, evil people always seem so happy. But we've gotta remember that those are only childish movies. In more adult movies, villains are always disturbed, bitter or maybe even miserable. Mr. Freeze in Batman, he was only pissed because they wouldn't let him have enough cash to save his wife. In the end of Batman and Robin, some of his goodness came back when Batman pleaded for his wife's case to be treated in the laboratory and succeeded. Evil never comes fully as a lifetime of happiness. Humans always start off good, no one is ever really born bad.
Guilt is what stops anyone from going bad. Guilt is the preventative measure instilled by God. Guilt towards bad deeds done to bad people can turn bad people into good people. That's why guilt is important, it may feel like an iron ball chained to your leg, but it's a good iron ball, that stops you running straight into hell. It helps you make the right decision in whether to be bad or good.