It removes the human from the actual thing. Which means two things: it's easier for humans to tell themselves that what they are doing is good for the world, and second it puts a power multiplier into the hands of potentially very radical people holding extreme minority views on human rights/dignity etc.

You just need one person to type "kill every firstborn and tell the parents that this is retribution for the rebellion" instead of that one person convincing thousands of people to follow their (unlawful) order, which might take decades of propaganda and indoctrination. And that one person only sees a clean progress bar instead of having to deal with 18-20 year olds being haunted by nightmares of crying parents and siblings, and baby blood on their uniforms.

Yes, in the end it's a human typing the command, but AI makes it easier than ever for individuals to wield large power unchecked. Most nations make it allowed and many also require soldiers to refuse unlawful orders (i.e. those instructing warcrimes).