Well, isn't that kind of like "an eye for an eye makes the whole world blind"? That never made sense to me. It seems to me, worst case, it would make the whole world one-eyed.
Not that I'm in favor of taking vengeance, but a well-started back fire will put out a fire. The metaphor doesn't work.
The promise (threat? observation?) of dying by the sword is not in itself a condemnation of ALL those who take up arms to defend their loved ones or even to punish wrong-doers. In a fallen world backfires and legally mediated punishment (note: NOT necessarily vengeance) sometimes are duties.
/pompous rant off
That is used to infer something other than what it means probably 100 times as often as it's ever used correctly.
Those who live by the sword are the "highway men" that steal and plunder. It does not apply to those who would use a sword (weapon) for self defence. Remember Jesus him self said if you have no sword, sell your cloak to buy a sword...(not that you all didn't know that, but I feel better for saying it anyways...)