If you think you are doing the deer a favor just make sure you are right! I say this because the deer may just be injured and could recover. About three years ago when I was doing some early scouting and squirlle huntingI saw a fork horn in september with the same situation except he always held his front leg up and limped on his back leg I assumed it was hit by a car looked like he was in alot of pain and could not move good. When I first saw him I had my .22 mag and thought about a quick shot to the head to end his suffering but the deer was able to get away so I did not. Then the next week I seen him again but he was far away and I noticed that he was putting his head down and eating. To make a long story short by the the time rut was on I seen him with a six pointer sparing in the feild and was walking fine. So my point is make sure your right in predicting the future and deciding that they will die, maybe they wont. Now for another side of my coin, If I saw a deer that had major and obious injuries like missing limbs that are all bloody or large open wounds with flies swarming around it I would probally shot it, But I would only shot it if anyone who seen the deer would say it was unsafe to eat and it would be obious I was doing the animal a favor, but again I am risking myself on others opinions if I am cought. But chances are if im in the woods deer season is always in so it wont be illegal. I only go squirrle hunting a few times before deer season, and I dont carry a gun in the summer so I don't have to worry about being tempted. Just make sure your right when you decide to end ones life.