In a ground combat scenario, autonomous weapons could be a good thing. Right now, soldiers are tasked with protect others as well as themselves, and in most situations the safest resolution is to kill the antagonist. A machine or robot wouldn't suffer from emotional lapses in judgment (anger, hostility). A robot may have better weapons skills, so instead of a kill shot, may only need to wound. A robot would be more willing to put itself in harms way to protect a living person.
The programming required for such a machine would be incredibly complex, but controllable with defined precision. A human soldier can't be controlled or programmed, and history shows that humans make a lot of bad decisions when it comes to the use of deadly force.
Power corrupts. And atomic power corrupts atomically.