Letting it learn is just new technology that is possible. Not bad on its own but it has so much potential to be used for good and evil.

But yes, its pretty bad if they are creating machines that learn how to kill people by themselves. Create enough of them and its unknown amount of mistakes and negligence from actually becoming localized "ai uprising". And if in the future they create some bigger ai to manage bunch of them handily, possibly delegate production to it too because its more efficient and cheaper that way, then its even bigger danger.

Ai doesnt even need sentience to do unintended stuff, when I have used chatgpt to help me create scripts it sometimes seems to kind of decide on its own to do something in certain way that i didnt request or add something stupid. Though its usually also kind of my own fault for not defining what i want properly, but mistake like that is also really easy to make and if we are talking about defining who we want the ai to kill it becomes really awful to even think about.

And if nothing happens and it all works exactly as planned, its kind of even bigger problem because then we have country(s) with really efficient, unfeeling and massproduceable soldiers that do 100% as ordered, will not retreat on their own and will not stop until told to do so. With current political rise of certain types of people all around the world, this is even more distressing.