When a meticulous hitman kills the wrong target, he has only two hours to figure out who betrayed him before the Mob takes its brutal revenge.

I'm wondering if it can be tighter. For instance, I don't think you need the word "brutal." If it's the mob, we're pretty much conditioned to expect brutality.

Same goes for "only" (it's assumed two hours is very short, given the situation).

Can it be just one hour instead of two? Not that this is a big deal, but it does make the count-down a little simpler.

Another suggestion, instead of making "meticulous hitman" the subject of the adverbal phrase, just make him the subject of the sentence itself: "a meticulous hitman kills the wrong target and has only two hours..."

I agree. After reading all your versions this one really hits it home. Great job!

I like this version the best.

You know I think this is really good. It's tight, gives us the main through thread, and hints at subplots which will involve him finding out what is going on. I can see you've done a lot of rewrites but to me this is the strongest.