Not so much the animals fear, but the chemicals made by that animals organs used to save its life can be extremely harmful chemicals at the point of its death. The adrenaline RUSH that enables the run or fight response is intened to save your life at all cost to the body.
People that can sense death in the air or area have written books over 100 years ago that the land near the large slaughter houses is the worse area a human could live on. The terror in the air from all the death/fear being the worse thing known.
I don't really know myself, but animals can love and hate just like we can. Growing up on a farm I know animals feel pain and death the same as a human feels pain and death. Organized religions like to down play animals as objects to be slaughtered and eaten at will, the animal always has the last laugh, because justice is served in the final days and always brings premature death not only to the human, but also their future generations. Nature always shortens the life of all creatures that go outside Nature's Laws.
It is intteresting how a person's pet can sense the person is ill or not feeling well and attmept to touch the human! All pets do this if they like their owner, even cows have feeling for their owners. I had a pet cow I called Number 15. She knew when I was near and always stuck her head up and looked for me. When she seen me she would push all the other cows out of her way and come to be near me, she also would be the first in line everyday to be milked and naturally she got a little extra feed for being a pet! One day I came home from school and she was gone, she didn't produce enough milk on her last test and Dad sent her to the market. I am sure this cow was frightened from the moment she steped 1 foot off the only home she knew. From that moment her glands were producing fight and flight chemicals and in her last 2-3 days of life her meat would have been saturated with these deadly chemials and her body quite diseases with her own poisons.
This is probably why so many books were written that wild game was better! Except those few deer hunters that chase a wounded deer a mile or those really sad people that eat road kill deer, these people eat the most tainted meats.
No matter how you slice it or flavor it, all meat is toxic and all second hand protein. Your eating a waste product. Then you have those that eat those creatures that live at the bottom of the ocean, the toxins these creatures have makes them as far away from being a human food as can be. The study of how and why animals/birds can eat the things they do and live, shows why nothing but an animal like they are should eat their dead bodies. Humans do not have the enzymes and chemicals that buzzards and hogs do, but we like to live like we do.