When a contract killer kills the wrong man he has only a few hours to find out how his flawlessly calculated hit went wrong and fix it before the Mob can take its brutal revenge.

When a meticulous hitman kills the wrong target, he has three hours to correct his mistake, before his cruel and sadistic mob boss exacts revenge on his own family.

"When a contract killer kills the wrong man he has only a few hours to find out how his flawlessly calculated hit went wrong and fix it before the Mob can take its brutal revenge."

I agree that "killer kills" is indeed awkward - you could go with assassin or hitman, or you could go with executes, hits, takes out, etc. Specifying how many hours is a good idea.

Wasn't so flawlessly calculated if it went wrong, was it? Doesn't make sense to refer to it that way, unless you add "supposedly," but that makes it kind of long and wordy?and this logline is already begging for a comma somewhere. Using "meticulous" earlier, to describe the man instead of his plan, thus implying his plan is meticulous also, is a good way to go, because you can then drop trying to say how it was supposed to go and focus on what he has to do now, as "fix it" doesn't say much, or mean much, or tell us much of anything. The guy's dead, no one can fix that. What he has to do, one supposes, is find out who's responsible for giving him the wrong target, thus setting him up as a target for killing the wrong guy, and make the person responsible the new target.

"Brutal revenge" is also vague, while giving the impression they have something specific in mind. So what are they going to do? Kill him, torture him, tickle him? To get out of creating this question, say something like "?before the Mob catches up with him." Also, bringing up the Mob at this point, when it wasn't mentioned before, is confusing. Maybe say he takes out a Mob boss by mistake, so we understand why the Mob is involved, and why they're not happy. The Mob getting revenge is a given, you don't have to say that, if you just say he's avoiding them until he can prove it wasn't his fault.

I'm not sure about the title. I like the idea of an alliteration, but, at least for me, it almost boarders on the comical, as a near tongue-twister.