The Looper business model. You're young and your prospects are limited. Working as a looper is high-paying, easy work in a Crapsack World. There's a catch but it's so far away you don't even care. Then one day your paycheck is a different color and it means there's a very special person under that bag on their head. You can take your money and run as far as you can but you know that someday, somehow,that person is you.
Old Seth literally falling apart as Young Seth is being mutilated alive.
Specifically, the scene where he's trying desperately to get to the place he knows they're going to kill him, but they aren't stopping at any point. One of his legs disappears while he's in the car, making him crash outside. As he hobbles towards the door, his other leg vanishes. By the time he's beating on the door begging them to stop in a tongueless moan, he's nothing but a head and torso.
And granted another layer of horror when Joe mentions, later, that he gains the memories of his changing past as it happens. Which means Old Seth would be remembering exactly what was being done to him, in real time.