Dan Savage wrote a book called Skipping Towards Gomorrah where he actively committed all of the seven deadly sins and reported on what they felt like. For his chapter on greed he went gambling, and what he wrote has changed my understanding of games of chance ever since. He argues that the appeal of gambling is not actually about greed - if it was people would try to find better ways to make money, since gambling is a terrible way to generate consistent profits - it's about the need for consequences. Most of the decisions you make every day don't matter: you can change your outfit before you go to work, order a different thing for lunch than you normally get, skip the gym or go the gym - whichever way you end up going, it won't really matter in the long term. But gambling provides constant consequences: every choice you make is either punished or rewarded almost immediately. There's a visceral thrill to that; we want to feel like we matter, and on some basic level gambling feeds into that egomania by suggesting we can control things that we probably can't.
Now, it might seem strange to introduce Looper by talking about gambling, because Looper is a movie about time travelers and hitmen, not poker. But consider this: almost every time travel story either affirms determinism or it affirms free will. That means that a time travel movie is either about a world where nothing you does matters because the future is already set, or else it's about a world where everything matters, because every choice you make has ramifications that echo into eternity. One of the more interesting facets about Looper is that it seems like it would take place in a deterministic world - after all, the idea of a "loop" presupposes an inescapable cycle - but the film is actually about free will.
And I don't just mean that in terms of how the time travel elements in the plot play out - although, yes, it's cosmology is elastic - I mean that one of the central themes of the movie is that actions have consequences. In a way, Looper reminds me of Memento, the movie where every scene was shown in reverse chronological order. Both those movies play with time as a gimmick, but you could remove the gimmick and the story would still work - it would just be a lot less interesting. If you took all the time traveling out of Looper you would still have a compelling story where a man who can't decide whether he wants to be selfish or do the right thing crashes into the life of a woman who is trying to raise her troubled son, but that version of the story wouldn't be as powerful. The fact that you get to see how the choices young Joe makes change future Joe's existence means that you get a clearer picture of what his actions actually mean; the malleability of the timeline allows the film to present things as literal truths that in most other circumstances would be metaphorical truths.
One of the hardest problems for evaluating morality in our world is that every alternate possibility to what actually happened is just theoretical; you can allege "well, if you hadn't done [x], then [y] would have happened, and that would have been better" - but the person you are arguing with is free to disagree with your supposition and to offer a completely different interpretation that makes them look better. Looper, however, allows for it's characters to see into alternate realities, so they can definitively say whether or not their decisions lead to the best possible outcome. That ends up grounding the movie's violence in a way that you rarely see onscreen; sure, per Hollywood's norm, in every scene where a gun is fired one shot is an instant kill and once a body is dead it basically disappears, but here the brutal acts continue to effect the characters long after they've been committed. When young Joe makes a selfish decision, he will end up answering to old Joe for it, because his sins in the present are what cause most of old Joe's problems in the future.
In the abstract, you would think that most movies are stories about how consequences have actions. In reality, that's not quite true. What would happen if that hero cop wasn't on the scene to stop the bank robbers? Well, some money would disappear and the insurance companies would be bummed, but life would go on. Semi-infamously, Raiders of the Lost Ark would turn out the exact same even if Indiana Jones wasn't involved, since the Nazis knew were on the trail of the ark before Jones showed up, and were defeated by the ark itself, not Jones. But there are movies where actions definitely have consequences, and for the most part they are morality plays. At the beginning, Looper doesn't seem like it would be a morality play, because when it starts the movie is too involved in building it's near-futuristic world and in dazzling the viewer with it's style to seem like it has much depth. However, as it moves out of the city and towards the Kansas farm where most of the action in the back half takes place you get farther away from the high concept stuff and towards a slower, quieter place. As you strip away the layers, it becomes less a film about big ideas about time travel and more about little ideas, like how important it is that you consider the needs of others before you do something that only benefits you. In other words, it becomes less about giving the viewer a big emotional jackpot, and more about giving them a sense that what they do matters.