It made sense to me.
If I was a leader of an advanced alien race and I had an army, I wouldn't want them to be too smart, because when I sent them off on obvious suicide missions or (wiping out every living thiing on a planet indiscriminantly including babies, puppies and kitties), I wouldn't want them coming after me if they refused. They were swimming in weapons, so it was likely they could do damage if they were ordered to.
If a malevolent alien race doesn't have a collective brain, then its going to have to have stupid "muscle" aliens to carry out its dirty work. The muscle would only kill when ordered to, however, to avoid them killing each other or accidently killing an ally (which is why they aren't wholesale killing humans).
If these stupid aliens end up in a place where they aren't having orders shouted at them, you get district 9.