I think "Responsible" raises some hard questions. Today, any weapons use by a robot might require a responsible human operator to first (remotely) pull the trigger. But what if an adversary allows their robots to fire without waiting for human permission? The first side to do so would gain a critical advantage, and we cannot have an autonomy gap.
Some have raised conerns that in the extreme case, this could lead to a world of machines anonymously killing without attribution. If you haven't, see Stuart Russell's compelling short film "Slaughterbots" [2].
[1] https://media.defense.gov/2019/Oct/31/2002204459/-1/-1/0/DIB...