depends on how you define "Slaughterbots" any micro drone with a small demo charge can kill a person if it detonates on the skull. Intel has a drone kit which can be pretty autonomous you should be able to train it to explode on heads, you might have a pretty big collateral damage as far as soccer balls and water melons go but it will definitely will detonate enough heads if released into the wild (within it's flight range ofc ;)).

A practical completely autonomous weapon as in one that makes actual decision is pretty far off, however with modern drones pretty much everything is already automated even the weapon release and guidance is machine assisted these days but that is just to help increase the accuracy of the strike as it can compensate for the delay.

The NSA probably already has them.