I’d like to point out that an FAI is still valuable as a weapon. You don’t have to choose between regard for life and military application.
An FAI could be tasked with weapon and material destruction only. A sufficiently fast FAI could dance amongst soldiers destroying rifles so fast that the soldiers would have no time to respond.
War is a game of tools. Reducing your opponent to sticks and stones can be accomplished without death. Possibly without even pain.