TV's Frink said:
But how is that revenge? When the loop is closed, the younger one gets a big payday and all his buddies celebrate with him.
JEDIT: How did you spoilerize that?

I guess it's not really much revenge at all. Ultimately "so that there would be a movie." probably works the best. I guess you could rationalize it as "he saw that a looper tried to kill him, so he closed all the loops to try to prevent that from happening again."

I googled and found an article with the director were he says:

“Or is he doing it because he’s come to power and he’s wiping everything out? It’s a good question.” says Johnson, suggesting there’s really no answer.

Really, most of the movie's logic doesn't work or really matter outside of the direct narrative (as is common with time travel movies). I do have a question though: They shoot old Joe's wife in the future after making such a big deal about how hard it is to kill in the future. Why didn't they just send her back in time to be killed?

To spoilerize: The text input here lacks an html editor, but still allows formatting from pasted sources. Just do the spoiler in a text editor (black highlighter) and paste it in.