What stops people from going around willfully killing others?

In some countries (mine included) life in jail constitutes 30 years and often there will be a parole period in the middle. Serious offenses like murder or rarely attract the full 30 years. In a society where people know they won't be put to death, and will eventually be released from jail ... what stops us from going around committing atrocities when we know we wont be very badly punished? Even though people say the world is getting worse in terms of violent behavior, when I consider that the punishments are quite soft, it seems that violent/ disturbing is still quite rare in comparison. So what stops the majority of us from doing crazy, messed up stuff?