I don't think any of the options are "inhumane". The boy is dead. I don't think they really have any remaining sense of consciousness. I crushed his head with my foot. A shot would be alarming and a waste of a bullet, and the wrench would be close contact (I assumed you wouldn't get bit if you used the wrench, a foot just seemed like the theoretically more practical choice.)
Clem just drew Lee by himself for me, IIRC......