One thing I’m not clear on from the movie is-
Why a Looper has to kill his older self. Wouldn’t it be more reliable to have someone else in the past kill them instead?
Is there anything behind this?
A Looper killing his older self signifies that his time as a Looper is done, he can retire. There's a big pay off, and they have about 30 years until they are found in the future, and sent back for execution. The significance of 30 years is of course as soon as time travel is invented. The future criminal organization ties up all loose ends, which includes loopers.
Loopers have an understanding that they aren't going to live until old age. They are paid well by essentially something like the mob. It is their job to make bodies disappear. Not the greatest line of work, but one that requires short term memory and thinking. They aren't thinking about working for 30 years in a 9-5 job like most normal people, they want to make money now, quickly, and to retire and enjoy it when they are young. This comes at the cost that they don't live to old age.
They aren't going to try to stop the system either when they are older, because if they do, their younger self won't get the money. A sticky time paradox.
I don't think it would be more reliable to have another looper kill off an older future looper. Those that are sent back are tied up with a white bag over their heads. They aren't supposed to be identified in any way by anyone. Another looper may not report that they just killed another looper's future self-- which again, they shouldn't be identifying bodies anyway.
The only thing that should be seen is the money, and how much. In these cases it's silver and gold bars-- which now and even in the future is a pretty basic form of currency. Specifically, it's money that isn't issued by any government and can't be traced. Silver bars for regular jobs, gold bars for the final job, their elder self.
In an interview with Slashfilm, the director, Rian Johnson, addresses this:
IT's important that you kill your own looper so that (a) you get the big pay day and retire and (b) your not constantly worrying about someone ELSE killing your future self.
If someone else did it you would (a) be trying to figure out how it is and stop them so that you can live (b) always be wondering if you can somehow avoid that fate. But if you close your own loop you have no one to stop but yourself, you have an incentive to allow yourself do it (the gold), and you will have certainty in our life.
Another reason is this way if the loop goes free, people in the present can go after you without concern that it was some other Looper who failed the job. Or better said, you dont have to worry about some other Looper messing up the job and them coming after present you.