One of the reasons the German's developed gas chambers is the soldiers psychologically broke down from killing people outright. The Nazis couldn't find enough people to kill Jews standing next to pits.

As we know, the gas chambers were horrifically efficient because few people saw the process. The same will apply to drone swarms. They will allow mass killings without each death making an emotional effect on the killer. So I agree with you, they need to be internationally outlawed in war like biological weapons. Otherwise, like you say, they will have the capability to kill as many people as the number of drones a factory can produce.