Dumb cop, dumb hit-man, dumb good guy, smart girl... (contains spoilers)
As every cop knows, whenever you go to investigate something you ALWAYS radio back to your base station to let them them know where you are and what you're doing and then radio back again after you finish, if the base station doesn't hear back from you within say 20 minutes they will try to contact you and if they can't will send a patrol car with at least 2 cops in it to investigate. And if anything should happen to them then 200 cops would very quickly lockdown the whole area. That would have been the end of the hit-man's 'career'.
And as every hit-man knows, if you kill a cop you had better start running because if you hang around all night - see the first paragraph.
And all hit-men (as well as good guys in a stand-off situation with a hit-man) know you need to be alert, sober, and in full control of your senses so as you can manage a standoff, the last thing you do is to open a bottle of whiskey and get drunk!
The only person who had any sense was the girl who after witnessing the murders took a photo of the hit-man which could be used as evidence, though a smart hit-man would have kept his mask on until he was well away from the cemetary and sure that no-one was following him or watching him.