What a stupidly extremist example, and you call what I said limited? This is completely different than the Uchiha situation anyway. First of all, only a number of Uchiha wanted to overthrow Hiruzen and their current regime. Second, the Uchiha were isolated and mistrusted, and as one of the two founders of the village that was now actively ostracizing you, the coup could even be called justified, at least to an extent. Third, the innocent Uchiha were never spared, Danzo made Itachi kill everyone, because that makes sense. Fourth, "5,000 people" is probably like 10 casualties at the most, since none of the Uchiha except Itachi could match either Hiruzen or Danzo, let alone all of the Anbu and Jounin that were going to fight as well. The village clearly had the superior manpower, to a point where they could just arrest all the plotting members of the clan which would, at most, result in a few militia deaths, none if Danzo and Hiruzen fight themselves. But nah - let's just make a child kill their family.
If I had to address that dumb example directly, then I'd say it like this - I'd true to dissuade him/her from doing it, or find out their reasoning, maybe those 5,000 people are trash. If I'm unsuccessful, I would simply prevent their attempt by not killing them, like physical restriction. I wouldn't kill my own kin.
Only idiots resort to extremes without considering every single option beforehand.