The two young gentlemen in the case (Brendon Fearon & Fred Barras) had broken into Mr Martin's house but to the best of my knowledge they were running away when Fred Barras was shot in the back (they were empty handed but either way they presented no danger to Mr Martin at the time of his shooting at them).
When someone is running away, the most logical thing to assume is that they are running to cover, at which point they will be able to shoot back.
The homeower is no doubt angry and thus may mess with his/her judgement and not realise that, rather then shooting them in the back for the logical reason they shoot them in the back due to anger or any other reason...
regardless, the correct action was taken..
If a random guy takes out a black piece of wood, he would be shot by armed police officers (if they were already there, and thought he may be armed)... just because he didnt have a weapon doesnt mean those officers were in the wrong... Thus any evidence taken corresponding to after the "incident" would be logically invalid.. i.e. it turns out he didnt have any weapons, therefore checking whether he even had a weapon is pointless unless he survives.
So saying they were unarmed is irrelevent, and that they presented no threat is also irrelevent. Using your logic, being shot in the chest or back makes no difference, and thus there would be no chase when anyone shot anyone that would be justified for any reason..
Besides, it's not really a likely scenario is it? When it comes down to it, most people don't have what it takes to kill someone in cold blood like that.
Putting aside my viewpoint that only a DEAD person poses no threat, some people would have a lot of fear and panic, they react instantly and dont think about it, that is not cold blooded, cold blooded is if you tied them up, then lit up a smoke, sat there while they freak out, finish the cig, and then shoot them