Some time ago I wrote an article for American Handgunner on whether an armed citizen should intervene on behalf of an innocent third party. I wasn’t exploring the thorny legal aspects of the issue but rather the moral/ethical ones, because the moral case for not intervening is rather stronger than usually realized. It got less hate mail than I expected.
I’ve also written a piece for AH that (as of now) is awaiting publication on the approach to and equipment necessary for intervening in an active shooter scenario, or escaping from one, which is obviously related to the issues discussed above. Of course your firearm is one of the pieces of kit that is critical in that scenario. That sort of situation is likely to require a long range shot, possibly well out to the limit, or out to your limit, with a handgun, and therefore a full-size sidearm is what you want to be carrying.
On the other hand personal protection is, we know, likely (but not always), to be necessary at much shorter ranges, and a smaller or just plain small gun is likely to suffice here (statistically speaking, of course).
Therefore the gun you choose to carry when you leave home is, or should be, made in part on the basis of a conscious choice about whether you are prepared to protect just yourself (or those in close proximity to you), or whether you will be prepared to protect the flock of sheep that most people are.
Which brings us back to the moral issues raised in the article referenced above.