The first point is that nobody likes having killing robots around.
Civilians really hate the idea. A basic reason this society accepted robots was the concept that "A robot will never harm a human being". As such, creating a robot that kills people would cause an uproar.
The military are more open to the idea of robots that kill people…as long as they are not their own people. That would be unacceptable.
You can hardly look more dumb than making a killing robot that then proceeds to kill your own troops. Plus, nobody would like the remote possibility of being among those.
However, no matter how you make the robots, there will be errors. You would need to teach them which people are ok to kill and which are not. And such robots would occasionally fail to notice the cues telling them that someone is a friend. (Not to mention that establishing the difference of friends vs foes will not be simple at all, since the enemy will logically try to look as an ally in front of the robot)
Then, in addition to the inevitable "mistakes" by the robot soldiers, there is the risk of sabotage. You bring your killer robot back to the base after an operation, just to discover that it got reprogrammed to start a killing spree on arrival. How could you trust that piece of metal with guns?
The second point is one of responsibility.
If a soldier kills someone, you blame the soldier. If a robot kills someone they shouldn't… who takes the blame? The company manufacturing them will not. Nor will those programming it. The blame (and target of legal proceedings) would probably fell on those that decided it was ok to produce slaying robots, along those choosing to deploy them on an operation, which means nobody really wants to take such a decision.
And it's bad as well when they work. If your troops protect a colony from raiders, they are heroes. If you just deployed some robots that did everything, the army gets much less prestige. So, even with no malfunctions, the army is interested in that attacks are only carried by people. It's such an important quest that it must be done by a man.
