"I liked you. I really liked you, Macoroni. But you know the rules." -- Tony le Stéphanois
Rififi includes the classic gangster movie scene where one guy has rats out his friend, and the boss of the gang finds him in a helpless situation. The squealer tries to plead his case, but the hoodlum mercilessly kills him anyways. In this case, the boss delivers a classic line before doing so.
Who would sign up for a job with rules like these? These rules seem counter-productive, since more people on your side will end up dead. But game theory predicts otherwise.
The situation is that you are held captive by a rival gang, and they want to know where one of your friends is hiding out. Here's the trade-off without "the rules": if you rat out your friend's location, you'll end up with a moral hangover but a better chance of getting off. With "the rules", if you rat out your friend's location, you'll less likely be killed by your enemies, but you will be killed by your friends.
If "the rules" are enforced 100% of the time, it will no longer be in your best interest to squeal. This equilibrium is better for your gang overall, since your rivals only can take one of your gang members hostage.
That's why enforcing the rules 100% of the time is so crucial for any gang lord, and that's why a ruthless, cold-blooded killer is so valued in that job market. Of course, once you bring torture into play, these basic rules dissolve, and things get even crazier. The take-home message is that if you don't grasp basic game theory, you will not be enjoying gangster movies as much as you should be.