We already have drones that kill and injure human beings. They're called missiles and torpedoes, and the third world has plenty of them too.
Also, there's this assumption that keeps getting made that humans will always make better decisions than machines. Personally, I think you're seriously overestimating the ability of scared, tired 18 year olds to make quick rational decisions under extreme pressure. Every single war ever, is littered with examples of human beings making mistakes, which end up with the deaths of allies or civilians, it's not even remotely rare.
And you know what, machines will make mistakes too, they'll kill their own side or civilians occasionally. But after they do that, their software can be improved and refined, over and over and over, with no limit. They will constantly and forever be improved.
A human soldier will not. There's only so much training you can give to someone in nine months before they're in combat.
And there's also the fact that machines are a helluva a lot less likely to think it's funny to piss on a Koran or to go rape some of the locals.