In the movie Looper, why do the criminal gangs only send victims back 20 years or so in time to be executed?

if you're looking for logical consistency in a movie about time travel, looper definitely isn't it, and I don't think there are any really airtight examples of one either, though I thought primer did the best job of that of any.

anyway, they sent these guys back, not just to be executed, but to to be executed by none other than their own former selves. presumably this was a way of creating a foolproof closure for the "loop" of their lives, which they'd agreed to in their contracts, not to mention convincing them once and for all there wasn't any way of getting out of it

sending them way the heck back, a long way, if that was even supposed to be possible in that movie, I don't know, could arguably be said to have been not such a great alternative, as there's precedent in these kinds of stories (a sound of thunder being the great example), not to mention in chaos theory (which is not something that was just made up for that jurassic park movie, it greatly affects the behavior of weather, the motion of fluids in general, and probably all of human behavior too, individually and collectively) to to the effect that any change they made way back there, no matter how small, could easily, and completely unpredictably, change what came afterwards, even all the way out to the end of time maybe.

but anyway, no, definitely not the most logically consistent time travel movie ever made in the first place. then again one of the reasons time travel (into the past anyway) is often considered impossible in the first place, is because of those logical paradoxes that are implied if you assume otherwise, so maybe there is no such thing as a really logical description of it in the first place, in reality, though I think you sure can come up with one in your imagination, if you assume other stuff like parallel realities and stuff like that to untangle the the lines of consequence. and stuff.