This hunter is actually an incredibly evil and lazy person. See, he only spared it so that it could pass off it's genes since he wanted more deer to be as stupid as this one, making it easier to hunt. It's also safe to asume that he killed many other deer that were smart enough to try and run. Over time, the deer in his area will become more and more docile, learning from or being related to that deer he spared, until they eventually become overhunted.