How deadly are they? As deadly as you want them to be; in the future's Volgan war (When the west goes to war with the fictional Russian-analog "Volgan Republic"), human soldiers are slowly replaced by robots created to fight wars. Slowly enough, in fact, that the ABC Warriors get more than a few kills in before the war ends, at which point they get to kill some more people while - at various times - rebelling against their makers, terraforming Mars, assassinating people for chaos magic rituals and getting involved in other unlikely scrapes.
Who's responsible? The military brains of the western alliance and the Volgans. Both sides came up with their own robot soldiers, giving them artificial intelligence, advanced weaponry and no immediately-obvious off switch.
Last seen... in a robotic mental asylum on Mars, reminiscing about their wartime struggles in 2000AD's current "The Volgan War" storyline.