Tony Martin shot the kid in the back. He then failed to do anything about informing the police, so that there could have been a chance of him being picked up and given medical treatment (and being arrested). That was what made it murder.

If he'd simply phoned up and said "I've shot a burglar in my house and he's staggered off wounded" there is virtually no possibility he'd have been charged for murder, even if the boy had died.