This is a theological question. After reading answers about why an abortion to save the mother’s life isn’t considered to be self-defense, the answer seems to be because self-defense is only ok in terms of an aggressor not mere threat. In other words, the child is innocent and not taking an aggressive motive.
If that is correct–that a threat to ones life is not sufficient to invoke killing in self-defense–then does this also apply to other scenarios? For example, if a retarded man is playing with a loaded gun which he clearly thinks is a toy and is about to pretend to shoot you, is it self-defense to shoot first? I’m sure there are other, perhaps better, examples… but you get the idea.
Good questions.
The answer, as I understand it, is that it is inappropriate to use deadly force (i.e., in self defense) against somebody who isn’t an aggressor.
In your example, however, the disabled man could possibly be considered an aggressor (depending on the circumstances) even though the aggression is unintentional. You don’t necessarily have any way of knowing his intent. The person who is acting in self-defense has to judge the situation as best as he can and make a judgement on whether deadly force is a appropriate to the circumstances. In your example I’d say ‘probably not,’ but I could imagine a situation where somebody with a mental disability (e.g., somebody suffering from schizophrenia) might be brandishing a weapon in a clearly aggressive manner that might justify deadly force in self defense.
I would characterize the abortion situation a little differently though. I see it as being more like this: imagine you are walking on the sidewalk in the winter. A car approaches, and the driver slips on some ice and starts careening toward you. Is it appropriate to shoot the driver?
I think the clear answer is ‘NO!’ The driver has done
absolutely nothing to harm you, they have simply lost control of their vehicle. In this case, you probably know without a doubt that the driver is not an aggressor, so you would be committing a terrible sin in killing them. It is the same with the child – we know without a doubt that the unborn child is not an aggressor, so deadly force in self-defense is inappropriate.
As for your example, I find it a bit extreme. I mean, why would a man who does not understand the concept of a gun have one, and more importantly, why do you happen to have a gun at the same time?
Your first question is fair. Your second, however…in the U.S., we have a right to carry firearms for our defense. Lots of people – even faithful, devout, obedient Catholics – ‘happen to have a gun’ at any given moment, since violent crime can and does happen at any given moment. I ‘happen to have a gun’ within reach right now, as a matter of fact

. I pray
every single day that I will never need to use a firearm in a defensive situation, but if one arises I am glad I live in a place that affords me a reasonable opportunity to defend myself and others against violent aggressors.
God bless you!