Everything that Z said.
Don't bombard a JW with facts. Slow and steady. Know when to say things and when to back off and let him think for a few days or weeks.
Try to reach his 'authentic self' — the reasonable person that resides behind the cult defensive wall. Nobody really believes that billions of good men, women and children deserve to be destroyed tomorrow just because they are unconvinced by the ramblings of an organisation that has got so many things wrong. Nobody really thinks it was a moral good for Joshua to follow god's instructions and commit infanticide. Try to reason with that person. Pick your moments. Be patient.
As for not going to war be content just to sow a few seeds of doubt. If your life was in imminent danger would he want armed police to save you? Apart from scale what's the ethical difference?