The title "how to head shoot a deer out hunting" is a little funny to me. It's pretty straight forward. Step 1: shoot the deer in the brain. Step 2 <---- there is no step two...
Personally I've never had one get shot in the jaw or nose, they've always dropped where they stood. However, if you hunt long enough you will eventually get an inhumane shot. I've had one. You try to be responsible and stack the odds in your favor but if you've ever missed (I have) you could instead have got an inhuman hit. I only attempt a head shot if I have a rest to set my rifle (this is most of the time) and I've never missed like this. But I missed a heart lung shot once with a black powder rifle because I didn't have enough experience with one (you have to hold solid follow though a LONG time, which you should do anyways but...) and I was shooting off hand. Not a problem shooting paper off a rest but I cleanly missed a deer this way. But that was a miss that could have been a painful wound. Imagine a .50 cal ball though the gut. That would really suck and could easily have happened. I was a grown man and an experienced hunter when that happened. I never fooled with a black powder muzzleloader again, it's not for me. I also had a miss when I was a boy due to a lot of bullet drop, not knowing my holdover, an underpowered rifle, and the real root cause ---> inexperience.
Those are two of my three misses. All due to inexperience. They all could have caused pain and suffering. The third "miss" probably did, and I'll never know. most longtime hunters can tell similar stories.
If we were seriously concerned with humane hunting we wouldn't use anything but the absolutely most effective hunting weapons. High powered scoped precision rifles. And we'd hunt up close over bait. <--- super reliable humane harvesting. But in reality that's too easy, it's boring and barely removed from farming, so we have an entire hunting season dedicated to primitives such as black powder and bow hunting. Because we're hunting. As people have done since before we were even really modern humans. And like every other apex predator.
While it's true that pain and suffering is always a risk, in my opinion it really isn't the end of the world. You try to mitigate that risk. You make a solid effort to be humane and responsible. But in my 30 years of hunting I've probably killed more biomass than the average wolf, and I'll bet the animals I've killed and eaten preferred going out my way then being killed and eaten by wolves. In the natural order of things, herbivores like deer are prey. Most starve, freeze or are eaten alive. Most experience a horrible painful death, naturally. If some small percentage of a hunter's kills aren't completely painless, I don't really see the problem with that. You try to be as humane and responsible as practical and accept there is always some risk. Unless you are a vegetarian, you are killing animals (directly or by proxy). I'm okay with eating meat.
