Think of it this way. When he's at his prime you can offer him a quick exit which is better than becoming feeble and dying of malnutrition or wild dogs, or hyperthermia, or a car at 55 mph. Death is a natural thing and he probably won't live past 7 years in the wild if he's smart and lucky. A good hit and he may never know what happened. Wait a few years and take him out while he still feels he's king of the woods.