Why is thief and robber criminal? Because it's the against the law. (Right! but why does the law prohibit it?)
The proper answer is that "if thief is permitted, soon there'll be nothing to steal in the world".
If you can jump a restaurant bill, there'll be no restaurant that serves you in the world. If you can always tell a lie, no one trusts you anymore. If you can kill someone you hate, you'll be killed someday and should be careful!
The point is not because it's against the law, but why it is against the law. The point is not how much you should be punished for thief, but what kind of punishment affects your behavior: If you don't want to be killed, killing should necessarily be prohibited and the killer should be forced to be in jail.
Reference: Schelling(1978), Micromotives and macrobehavior, pp33, "woodcutter and bystanders"