OK here's is my take on all this...
Someone breaks into your house and your children are sleeping. You don't know if the person(s) breaking in are criminals or family/friends playing a prank or whatever the case might be. Would you shoot first before finding out who and why they are there? Come on an get real. Yes there is a risk to you and your family if you shout out, hide or sneak down the hallway to see what's happening. There is an even greater risk if you shoot at the person(s) since they may be armed and fire back at you and your family in their self defense. Or maybe you kill some pre-teens and have to live with what you did for the rest of your life. Who knows. Would you really shoot before knowing the facts?
I have a 4 yr old daughter and a wife I would die for in a heartbeat. If we were leathally threatened, of course I would kill to protect them and yes I have plenty of firepower. I can't stand all the assholes that think "i have a right to kill because they COULD be a threat". The word "could" or simular doesn't cut it people. That's an excuse to kill, not a reason. It's self justification to do an evil act. Just because you are afraid of it, it doesn't mean you have a right to kill it. Think of going out in the bush and seeing a Grizzly Bear wandering around close to you, yet not threating you. If it becomes a threat, do what you have to do to it. If it's not intending harm or you don't know if it is, leave it the hell alone until you find out otherwise. Attacking it might get "you" or someone else killed as well.
Back in my teen years I used to break in to homes for food since I grew up in an extremely poor household... I think if someone killed me and found out "after the fact" the reasons and intent, and that I had no wepaons, they would feel guily as hell. At least any human with a heart would. Would you feel justified after the fact? Again, just because someone is in your home and it's dark out, it does NOT give you the right to take a life before finding out what the INTENT is. Grow up people.
This is not political or religious reasoning in any shape or form, it's common sence and we all should know better. If you jusity killing because of fearing the unknown, you need some help and should not own a firearm. Plain an simple.
As for the original question, there is no way I could kill without and ounce guilt. Even if I killed an attempted murderer that came in and started shooting at me, I would still wish I didn't have to do it, but would not hesitate in killing the person in defense. Remember, all people were children at one time and they all have or had mothers and fathers, and possibly siblings. It would still hurt knowing a human life was taken by my hands, even though I knew I had to do so. All humans with a conscience would feel it, no matter why and who they had to kill. It's called "being human". For those that say they would have no guilt what-so-ever, you are full of shit unless you are one cold blooded, blood thirsty asshole...