I saw an interesting discussion about this some years ago.

One of the person involved pointed out that yes, it does feel worrying to think that a soulless machine may not feel anything while killing people.

But on the other hand, if robots were used in place of people, the nanking massacre[0] would likely not have happened. It has historically required humans to really fuck up things.

[0] http://en.wikipedia.org/wiki/Nanking_Massacre