Legally, how should they have handled it?

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From a legal standpoint I really can't say. However taken as a real life scenario in that place and at that time, I seriously doubt that they would have ever saw the inside of a court room had they turned themselves in, as Drew wished for them to do.

Even in more recent times, I have heard of incidents of people travelling to some of these backwoods type towns, and entire families disappearing, never to be heard from again 

Putting it in a nutshell they were legally obliged to report the incident. However moving the body would have been tampering with a crime/investigative scene and that you can't do. It's against the law and also calls into question the veracity of the justifiable homicide.

If you dip your foot into a pool of piranhas don't cry when you lose a toe!

They handled it pretty stupidly by today's standards. A medical examiner could determine that Bobby was sodomized by that specific hillbilly, which is probably all the evidence the defense would need to exonerate their act as a justifiable homicide (that and the gun they recovered). However, so far as I know this sort of DNA profiling wasn't developed until the 1980s. Back in the 60s and 70s, a doctor could probably have determined Bobby was raped, but not necessarily by Mr. Bill McKinney. But even given that ambiguity, I would've taken my chances with the authorities. If the dead guy's momma was sitting in the jury box, surely a good lawyer could've successfully argued to move the trial to a different city.

Is the OP by any chance, asking for a "friend"?

it is tricky because they shot the guy in the back. Theyd have a tough case here given the fact. A good lawyer would defend them and say they did so to have an advantage over the hillbillies (ie gun vs bow). As far as the logistics, theyd leave the body where it was, try to make a marker of some sort and head back. Try to leave the crime scene untouched im guessing.

They all know the law is broken and they have no proof that the man was killed in self defence. They would most likely all go to jail if they went to the police.

They buried the body in the bogs around the river and the whole lowlands area was gonna be flooded in a few weeks for a water supply/reservoir...they even went there to canoe and hunt it one last time before a dam was finished
I doubt anyone even knew the body was there...and it would have been unrecognizable a few months later ...
They seemed like they were gonna keep their mouths shut so...no prosecution