Life imprisonment is a bit harsh, but 5 years sounds about right. This clearly was not murder. Without pre-meditation it isn't murder. Neither was it a crime of passion where the individual knew they were shooting at a human being and intended to do so, but did not plan it.
Instead this was a crime of pure negligence in which a human being was shot and killed. The basic facts are, they were out in a group of at least four people. All persons were not back to the group yet, thus there was still another guy out there. The sun was down and it was dark. CLoud cover made it darker still. There is no excuse for loading and firing a gun after dark. He only saw eyes and failed to identiy his target. With one more hunter in the group yet in the field, poor light, etc. taking a shot at eyes is just plain stupid. How many deer have you ever seen approaching an area with bright headlights and swamped with the smell of deer blood?
The guy was criminally begligent, at least as bad as if he had commited vehicular manslaughter. Similar punishment should apply.