RE: Best way to finish off a deer w/knife??
First off, I'd only do this as a last resort. I would think there must be or might be some way of rigging a knife or other stabbing/cutting instrument so one could perform the coup-de-grace from a safe distance ...like say 4-6 feet.
If I was place in a situation where I had only a hand held knife with which to put a deer down, I wouldn't consider it unless a 6' long stick could be poked into or onto the still living deer with little response. A small deer with no headgear is one thing. A decent sized 4 point is quite another matter. Wisdom and prudence are required here.