Feelings do matter into it, actually. Especially when you convey those feelings to the police (or more intelligently, your lawyer).
There is a huge difference between indifference to the slaying of an intruder then saying, "Yeah. I was in fear for my life. I did what I had to do, right?" and giving your statement as a nearly shattered person by this tragedy, and expressing how all you could do was think of yourself/friends/family laying dead/mangled/raped on the floor of YOUR OWN HOME, and that you had to remove that threat from the situation.
Believe me, come off as the victim, and people will treat you like one. Act like a Rambo, and they'll start nosing around the "details" of the event.
As for "when the threat is neutralized", there really isnt too much to talk about. When that person isnt going to get up again, the threat is neutralized. I busted my brother's nose (with a backhand) trying to GET AWAY from him when he was messing with me years back. We were just kidding around, but I still injured him while retreating.
A perp is still a threat as long as they are mobile. If you need to empty your rounds, so be it. Remember point #1 - emotion wins the case. If you need to just out and clip the guy, remember point #1, and an extra tidbit - A headshot is most commonly pulled off during a struggle for the gun. A struggle. For. The. Gun.
Also keep in mind that any time your whack a guy, there can always be blowback. Civil crap, legal crap, even his friends coming after you. As for the civil battles, I really have no experience with that. The legal issues - you now have two new tips (Tip #1 - Emotion. Tip #2 - Headshot Struggle) to help you in that department.
...As for the last one - You remembered to reload, right?