He had told people beforehand that he intended to kill anyone trespasasing on his land, and the teenager was shot outside the house as he was running away. In law, I'm afraid, that made any plea of self-defence quite meaningless - and so ilt should.

He should have been disarmed based upon his intent to shoot to kill any trespassers.

Even in the US you can't shoot someone if they are no immediate, real, and deadly threat to you -- and even if someone has hacked your family to death and is now running away, you cannot shoot them dead (although in this case you might very well be acquitted if you did). The threat must be real, immediate, and threaten deadly harm to you or another (and that last has to also be real and immediate). There is also "discrepancy of force" -- an old woman in a wheelchair would be under less restriction in the use of deadly force if she shot an attacking 20 year old male in good health, for example.

Nope, even in the US you cann't shoot a fleeing intruder in the back.