Your first line of defense with a gun is threatening to use it, not using it.
But what else can you do in a situation where your life could be in immediate danger, and you have no way of safely assessing the danger? Do you really think that shooting first and asking questions later should never be an option for something?
Also, why on earth should someone stupid enough to break into someone's house knowing that their life could be in danger be given any slack in this situation?
That would be like pressing criminal charges on the owners of a railway because some drunk idiot wandered onto the tracks and got killed by a train. If you're dumb enough to put yourself in that danger, you can't expect to be exempt from the consequences.
Did I say that?
If my "slack" you mean "not be lethally shot", and you still mean to ask this question, then I don't think I can answer it satisfactorily. Merely breaking into somebody's house does not forfeit your right to live. If you shoot to kill and do kill somebody who breaks into your house but shows no indication of actually harming you, then you should be charged, because that's not self-defense.
The only time it's justifiable to kill another human being is if your own life is in imminent danger or if killing that person would prevent an undeniably and significantly greater amount of harm, such as assassinating a dictator if it's impossible to actually secure him in a more humane manner. The fact of the matter is that not every breaking and entering case is a case of life threatening imminent danger.
It seems reasonable for a criminal to expect his life to be in danger if he breaks into someone's home.
If you shoot to kill and do kill somebody who breaks into your house but shows no indication of actually harming you, then you should be charged, because that's not self-defense.
Your first line of defense with a gun is threatening to use it, not using it.