I have followed this story with great interest since the day it happened. I had a suspicion of the way it would turn out, and time has proven that suspicion!
I wonder if the decision might have been different if he had either stabbed him to death or used some other means to kill him?
I wonder what would have happened, if the burglars had inadvertently killed their victim?
As the odds of being caught and convicted for murder sometimes appear low in the UK, perhaps his biggest mistake simply wasn't killing them both?
Regarding shooting someone pleading for mercy. Enter my house uninvited in the middle of the night and it will all be over before you get a chance to utter the first word in defence!