What I don't understand, is why the OP and his partner "left" the animal??
Did they forget to bring their matches to build a fire??
Were they dressed improperly?
Did they forget to bring a day pack, with an emergency blanket to keep off the dew/rain etc?
Maybe they were afraid of the dark??
I've put several animals on the ground, in all sorts of weather, right at the close of day, without any hope of getting it out until the next morning.
That doesn't mean I was going to leave it for the wolves/bears/coyotes etc.
One Moose that was taken beside the Prophet River, during a mid October hunt, was shot around 6pm, about 20 minutes before dark. We were almost a Klik from the nearest road and maybe three kliks from camp.
Between us, we had two emergencey blankets, a couple of cans of peaches, matches and TEA BAGS.
The very first thing we did was to gut the critter, set aside the liver/heart/tongue.
The second thing we did was get as much reasonably dry firewood as we could gather, before the batteries in our flashlights ran down. Luckily, were were right on the fringe of a two year old clear cut/brush area and there was plenty of wood to burn as well as smaller stuff to use for a shelter frame, ground cover to place one of the emergency blankets over. The other blanket was used as a cover/heat reflector just far enough away from the fire so it wouldn't melt or catch sparks.
Then, around 10pm we sat down to a great dinner of flame roasted liver and heart strips, followed by a desert of canned peaches. Life could be a lot worse.
One thing about the woods up there, they often have a thick carpet of moss. The problem with that moss is it's DAMP. That's why one blanket was ground cover.
The other issue with this moss is that it dries very quickly and when fire gets into it, it starts to burn and won't stop.
That meant one of us had to stay awake to make sure the fire didn't get away on us.
Well it wouldn't have anyway. The temperature dropped from +10C to -10C by 3am and the wind came up, driving heavy snow. By 5am there was over 10cm of wet snow.
Anyone that knows the Prophet River area of BC knows exactly the weather change I'm describing.
Luckily most of the leaves were gone from the trees or it would have been a nasty situation.
No, we weren't snug and warm, but we were dry and not so cold that we were shivering. Luckily, our clothes were layered and we had slickers in our packs. The slickers were used as blankets aroung our legs, up to our coats. We piled moss over the slickers and we were OK.
The wood we had gathered made things much more comfortable.
When daylight came, we got the fire hot enough to to melt snow in the empty peach cans and made Tea.
After that, it was time to take the hide off the moose and debone it. First the hind quarters and back straps, which we loaded onto our pack frames. Then we headed for the road.
Dropped the meat beside the road and made a return trip for the rest of the meat. We deboned the front quarters and took off the tenderloins and put them into our packs, along with the liver/heart.
Before we headed back, we took pics of the kill site and checked out the gut pile, which was appx a hundred meters from our makeshift camp. There weren't any bear tracks but wolf tracks were all over the area and they had eaten a bunch of the guts.
In that country, a rifle shot is as good as ringing a dinner bell for the scavengers/predators. There were dozens of ravens in the trees as well, waiting their turn in the pecking order.
Unless there is a very good reason to leave a kill, stick with it, until morning. That was a golden rule I learned close to 60 year ago and its never let me down.
A good hunter is a prepared hunter. It doesn't take very much to carry enough basics to spend a night in the woods or whatever. Just make sure you know what nature can throw at you and what you can handle.
If you can't bear to spend the night out, or are afraid of the dark, taking an animal that close to dark, is not ethical IMHO.