MQ, ethics is a personal thing. what some hunters think is ethical, others don' t.
I would have taken the shot. after he got out 10yds or so away from the tree, I' d have shot him a little forward of midway of his back. at that angle you would have had a good chance of takin out one lung and maybe even his heart. either way you should have gotten a complete passthrough, which should have left a good blood trail.