Not because the concept would be wrong or there would be no need for it - of course a state has to be able to defend its population against attacks - but because as soon as there is a situation where it applies in war, both sides seem to stretch it to absolutely unrecognizable lengths and use it to justify essentially everything in warfare.
That's why I'm wary if someone makes a theoretical argument about personal self-defense that's tailor-made to justify killing. It feels too much like the same tactics in war propaganda.
There are nonlethal ways of defending oneself or others, too, btw. Learn martial arts, knock them out, use a taser if you have too, then grab your kid and run. None of that requires shooting them.