I take the points you make, and have great respect for your experience in the Armed Forces.
But what's the moral and legal distinction between what this soldier did and the killing of Osama bin Laden?
Should the American government be on trial for cold-bloodedly murdering bin Laden when they had him cornered and he was defenceless? Was bin Laden entitled to justice, notorious though he may have been? Was his notoriety in itself a justification for his treatment?
Can we equate what this soldier did to the completely unprovoked killing of Lee Rigby, for example?
Without having attended the trial of Marine A and heard the evidence, I don't feel qualified to comment on the verdict nor on the tariff that should be attached to the life sentence.
I see your points. Yes, his capture would have been legally preferable and was probably specifically ordered but then you don't send in the army and have high hopes of capturing people. Rules of engagement for armies everywhere are that if someone has a gun or you legitimately think he has a gun and you or someone else on your side or innocent is likely to be injured then you fire first.
There's also a difference between an armed engagement scenario involving building-by-building combat and a situation where an enemy combatant has been captured and is in no way a threat. The former is probably a soldier's worst nightmare for assault as it's so easy to take casualties, even if you have overwhelming force. The latter has no precedent or legal support for murder.
In short, if I were a soldier tasked with capturing Bin Laden, I went into a room in an authorised combat situation and felt in the slightest threatened by someone who is a known bad-guy then my training would kick in and the bad guy would be dead. If I were a soldier who captured an enemy combatant then training is very specific that he's de-powered and detained with urgent care called for serious injuries to him.
Each soldier knows his rules of engagement, they differ for every theatre they're in. It's something that's beaten into you. For example, the yellow card for NI, the theatre specific ones for warzones, UN situations, etc.
The Lee Rigby scenario is also not related. That was a pre-meditated murder that would have taken in the first likely squaddie that they found.
There's a genuine point of debate over the role of the army in certain areas. The army is trained to be peacemakers at best, they're certainly not trained to be peacekeepers, the marines and other semi-special forces are even less likely to be trained to be peacemakers or peacekeepers. If sent on UN duties, army units are there for their threat of retribution rather than their genuine peacekeeping skills. In an ideal world, there would be an interim peacekeeping paramilitary force, essentially a hybrid between police and army, with training as heavily armed peacekeepers capable of looking after themselves and backed up by heavy duty army support when needed. The current army structure is about as valid for peacekeeping duties as a WW1 dreadnaught would be in a modern navy.