If I were put into a situation where I had to use force to protect myself and kill someone, it would not cause me everlasting emotion problems. It's also a lot better than being dead or otherwise. If I had to kill someone in self-defense, I would, guns or no guns (that's not really even part of the issue is it). I wouldn't feel be overly affected about feeling bad for the guy afterwards.
I suppose if you cannot reason that out and have emotional problems with it that's one thing, but to me it's just a logical conclusion that I had no choice. The only thing that would bother me some is just the scariness of the whole situation, I wouldn't be bothered that I was the one who came out on top.
By the way, keep in mind I said I do not own a gun, but I support the right to own one. If you want to buy into the perfect world where "civilized countries" don't needs guns, feel free, but I find that to be naive. There's no such thing as a utopia. In a practical world, you need weapons.
I don't disagree that there are some side-effects, some people who freak out and have a gun available and use it. Domestic violence, road rage, whatever.. but there's always trade-offs, nothing is 100%. Do you outlaw alchohol because in a perfect world no one "needs" it? It causes drunk driving accidents which kills more people probably than guns. Why not? Because we like our freedom to enjoy alcoholic beverages! And because in the US when they outlawed alcohol it became a huge criminal enterprise which funded criminal organizations... (and people still drank)