Oliver Wendell Holmes summed it up perfectly when he said (and I paraphrase slightly, I think): "Law is made for the bad man." Absolute genius. That is, laws are designed to channel human instinct/motivation to achieve whatever goals you want to reach.

Of course, "good" and "bad" must be defined externally to the law, but once those are determined, laws need should be designed to reach those objectives. "Good" men don't need an external set of rules with associated punishments (laws) to do "good," because they're "good." But the "bad man's" badness must be counteracted with a set of rules which provide incentives/punishments for bad behavior, thereby encouraging him to be good. Law is therefore made for the bad man.