There is what is on paper, and then there is real life. Real life is when we are forced to make decisions on the fly, and do not have in our possession "the trustystate game handbook" that wouldn't cover this topic anyway. I have a simple rule that I try to follow in these situations, that may or may not help. Do what you feel is the right thing to do. Since some will feel different than others, there will be no right, or wrong answers.
Like I said before, I would have killed him right when I found him, to end his suffering. Since I didn't havea gun or bow it wasn't hunting, so I would behead him and mount his head on the handlebars and go about my day with my new friend.