The whole premise of the movie is based on one fact i.e. you cannot kill people in the future, but the whole story is based on the fact that the old joe comes back in time willingly to avenge/avert/avoid the death of his beloved wife IN THE FUTURE ... does this make any sense?

If the mercenaries in the future can kill Joe's wife and get away with that, why can't they kill all other they wanna kill, why have a looper setup in the first place?