Actually, I saw a drunk guy at a service station once and took his keys. There is no way I would sit idly by and watch a clearly drunk person drive away if I had the power to easily stop him.
This sort of reminds me of what happened in the movie Spiderman when he let the theif go. He paid later, but it could have been someone else just as easily.
It's a crime to roll a drunk, what else did you take?