Well good. Try him for murder then. Or rather manslaughter or something like that since he killed someone in the middle of a fight. We try 15 year olds for that all the time.
What you don't do, however, is what we did here. If we do, then we violated this guy's civil rights and he's owed compensation from us as a result.
1) As I said before, soldiers don't get trials. They just get POW camp.
And if they are not treated in accordance with international conventions on the treatment of POWs (as in this case), then they have a right to redress for that abuse.
2) There's no doubt of this guy's guilt in the first place.
Western law explicitly does not allow such claims, except from the defendant (who can choose to plead guilty). In the absence of such a plea, he must in law be held to be innocent until his guilt is established in court. If he is treated as though he were guilty, without such a trial, then he has a right to redresss for that abuse.
3) Manslaughter means killed by accident. That's not what happened here. If you're going to go the civilian route it's first degree murder.
That's for a court to determine. But in a situation where two groups are armed and seeking each other with intent to kill, there is a strong case for a preemptive attack to be considered self defence; had he not killed, he had good reason to believe that his life was thereby put in danger. Justified self defence is not a crime; if the court determined that excessive force was used in self defence, or that his intent was not lethal, then manslaughter could well be the determination; but all of this is hypothetical unless and until there is a trial. No trial, no guilt. That's fundamental in western law.
As the question of charges is only relevant when he is being tried in accordance with the law; and as he was instead unlawfully detained, he is entitled to redress for that abrogation of his rights.
The law doesn't care what might have been if only the enforcement had been done in accordance with the law; it cares about whether or not an individual's rights were denied - and if they were, that individual is entitled to redress, regardless of any other circumstances - the other circumstances must be dealt with separately.
The guy is entitled to compensation for his mistreatment. Whether or not he should also be charged with some offence. However it seems that the actions of those who denied him his rights have seriously compromised any possible case for murder or manslaughter against him - and that's not his problem, nor his fault.
In short, if you are unhappy with the way this case has turned out, it is the US authorities, and their deliberate flouting of the law, that you need to blame. Had they acted lawfully, the person in question might well be serving a life sentence for manslaughter or murder. But they broke the law, and they must accept the consequences.
This is what sets civilisation apart from barbarity - in civilised places, the authorities are not above the law, and are required to comply with it at all times; even when dealing with people whose guilt appears to be beyond doubt.