Who's fault would it be?

[Technoid]

This is something that tends to trip me up, knowing who's guilty of what and who's fault it is that something happened. Let's see if I can explain...

Let's say you know someone who's going to drive while he's drunk tonight, and has a history of doing so. He's in another city, and you really don't like the guy and don't want to call the police and get involved in yet another one of his messes. So he drives when he's drunk, and wrecks and kills somebody else. If you had just called the police, he wouldn't have wrecked and the other guy would still be alive. Is that your fault?

If you called the police, would you just be saving him from himself again, when you should just let him drown and learn from his own mistakes? I'm not sure if you would in this specific example, but let's say if you had gotten involved you would've just been in as much of a mess as him. Is it then reasonable to have not called the police, which then caused the other guy's death? You didn't drive the car yourself, you didn't put a gun to the other guy's head and shoot him. But if you had simply called the police, the other guy would still be alive. How would you like it if someone let some guy crash into you and kill your whole family just because he didn't want to get involved in the drunk's problems?