After its over, its just beginning
You will most likely have nightmares. You might need some counseling to cope. It's ok. Someone whom that won't weigh on their mind is probably a little sociopathic. It would be normal to be messed up afterwords but consider the alternative.
The aftermath.
Some folks have an easier time of this than others. Warrior types (cops, military, emergency, etc) have sort of a built in defense mechanism and support structure within their culture that allows them to function fairly well immediately following an event and for a while after. They also have a lot of experience contemplating, training, or actually dealing with conflict and have usually answered this "Could I?" question to a higher degree than most.
I am totally generalizing here of course. After a trauma, in these professions and others like them, they are able to "suspend" their feelings about it in order to do what they need to. When things settle down is when they actually "deal with it". Paperwork, debriefing, then finally a few moments alone to sit and think about it. Then off for a beer with a close friend or two that is "in the business".
These professions, and the culture they create have institutionalized attitudes and traditions, even ceremonies, going back a long time that help their members cope. They wouldn't exist if this wasn't a "big deal".
I brought all of this stuff up is because your average civilian doesnt really have any of the above support or institutionalized attitudes. Friends and family are only so much help because they "haven't been there" so to speak. Sometimes a member of one of these "warrior professions" will step up and take a civilian aside for a talk but there usually isn't a lot of common ground. Sometimes its a complete culture clash. (Thanks to all of the Military, LE, Paramedics, Fireman etc that take the time to do this, its clearly not easy). The civilian is left mostly to themselves and people who have never "been there" to help them. Pretty sad. Even if they can afford therapy.
This is topical because one of the toughest emotional obstacles for someone going through the post trauma of a defensive situation is guilt, real or inflated, and all of the associated self recrimination. "Why me?", "What should I have done different?", "Why did I ever get that gun in the first place?", "Am I a killer now?", "How can I face my friends and family? They will never look at me the same again.", etc.
Having pre-decided your decisions about this stuff and being "OK and resolved" to your place in a defensive situation can go along way in getting through the aftermath because it directly addresses the biggest emotional obstacle, guilt.
Acting with purpose and unapologetic acceptance of the situation is likely to reduce your risk of finding yourself asking "What they hell was I thinking?" because you know exactly what you were thinking and you were OK with it. I believe that it will also help you be OK with it afterward too.
So my thinking is that, in addition to helping you stave off panic and function better in a defensive situation, being "OK and resolved" to your place in a defensive situation can also help you get through the aftermath.
For a civilian (as in non-warrior types) with very little institutionalized and cultural support, I view answering the "could I?" question as thoroughly and realistically as possible as paramount.
There are new reasons to seriously address the "could I?" question coming up as this thread progresses. Agree with them or not, does anyone really see a downside to really exploring it?