By that sort of reasoning, when a nation rolls tanks into your borders, you should just let them do it because more people would die if you fight back.
If a nation actually did that (release a bioweapon and pre-immunize their own citizens)... well World War is probably overselling it, but I could certainly imagine contained conflicts, sinking of cargo vessels, shooting down of planes, targeted assassinations... etc. It's very likely that every nation would have highly vested interests in making sure that whoever authorized that weapon was removed from this planet.
