Well in this situation, he had kicked on the bedroom door and then he fired the killing shot. You could threaten the intruder with the gun without actually killing them. You could fire a non lethal shot, but even then as you said yourself people get terrified and it would be very easy to misfire and kill. You could attempt more communication beyond yelling out once before going to murder.

You're dealing with human life. It's absolute, and the death penalty isn't even given to some of the worse crimes let alone unarmed breaking and entering. To balance the fact that no amount of law will save someone who is in imminent threat of death, it allows you to use self defense in situations where the criminals acts wouldn't warrant death.

But do you really think kicking on someones door like that constitutes immediate threat of harm or death? In fairness it is close, but as I pointed out there are other options in that instance. If the situation had extended to the door being knocked down, and he was right there about to hurt you, then yes I would agree. But it is too distant to warrant self defense - there was no immediate threat, the door was still locked and not open.

It may seem like I am splitting hairs, but when it is life and death the law should and does split hairs. If you allow people, who have options beyond killing someone to help the situation, to be justified under self defense, it becomes a slippery slope. The law is already lenient in giving the self defense justification for murder, considering death is such a huge punishment for some crimes which are pale in comparison.

And this isn't just me being favorable to criminals, it is how it works in most places. That is why I was asking how the US justifys having lenient self defense laws - perhaps I have just been mislead about how lenient they are from all of the people on Reddit saying how trigger happy they would be in these situations.