I don't think it's so much the "most inhumane weapon", but how inhumanely a weapon is used. Whether it's firebombing a city with napalm or sticking a bayonette into a child..

I may be biased of course but I think the most inhumane *usage* of a weapon was Churchill's desire to incinerate the German people. Sure the Nazi's tried to do the same but they were self-proclaimed "evil" people. Churchill & Co. were supposed to represent democracy and human rights, so why was he so keen on mass murder? Not to mention his (failed) attempt to drop poisons and chemicals on Germany.. thankfully the RAF convinced him incindiary bombs did more damage.