AGAIN SPOILERS DO NOT READ IF NOT YET SEEN THE MOVIE...



The younger Joe was in a paradox in which he couldn't kill the Rainmaker himself, because he grown found to the child, but at the same time knew his older self would had stop at nothing to kill him. I knew he wouldn't be closing the loop by killing his old self, because he'll still find and marry his Asian wife and she still will get killed courtesy of the Rainmaker's goons. Therefore he had no choice BUT to kill his (younger) self!

I think the scene way early in the film where he kills his older self AS PLANNED was the most UNDERRATED part of the film. He killed his old self and yet his wife still gets killed by the Rainmaker's goons. No matter what he could have done, with exception of killing the Rainmaker, his future wife still would of gotten killed.

By killing himself, he not only save the life of his wife in the future, but he (he hopes or gambles with) sacrificed his life for this CHILD. Like I said, this was ONLY a gamble, because the film WISELY left it ambiguous in actually showing how the child actually BECOMES the Rainmaker! Throughout the movie you only got suspicions and assumptions of not only about his origin, but about his future as well. However, showing that not all Loopers are evil through this sacrifice, Joe gambles with hopes this will not make Cid transform into the person that he's depicted as becoming. Of course, we never will know this, which is why it was smart leaving BOTH his future and his origin ambiguous, because quite possibly, the events that happened PRIOR to meeting Joe would of made Cid to transform into the Rainmaker anyway!