The witnesses would quickly speak against any genocide going on today or in the past caused by men. When it comes to God, their eyes glaze over and they don't imagine what it would really be like.
It is good to bring up an example of genocide to them, Nazi, Germany, Rwanda, etc. and ask if they agreed with those. Then remind them that this genocide they preach about would be on a much more massive scale than any in the past or present.
They may not seem to hear, but later it may surface in their thoughts and force them to actually picture such a horrific scene. It helped me.