Since so many soldiers go thousands of miles to put themselves in these kill or be killed situations, there really is a deliberate choice involved. Feelings of guilt in these situations are perfectly rational. Feelings of guilt rooted in self-questioning whether someone made a mistake are not quite the same thing as the vague idea that emo is a sign of goodness I think.
So far as the discussion bears on Snow White, Snow White knows that the return of Cora's heart has a real chance of thwarting Cora's attack (otherwise it would have been dismissed by Regina as a foolish ploy,) and
that Regina would be enraged and pose the same kind of magical threat Cora did. (No, Regina, despite cooperating with Cora, was not the same kind of threat, or Snow White wouldn't have lived long enough to swindle Regina.) The supposed need to kill Cora is contradicted by deliberately creating a new enemy.
Snow White being all emo about it doesn't change that. Trying to back out? Well, if Regina in the follow up scene rips out Snow White's heart, starts to crush it, then tries to stop herself just a wee bit too late, well, who will feel Regina's newborn goodness is vindicated?