As others have said, I think this would be one of the biggest factors when it comes to a traditional lethal injection. Truth be told, reading through a lot of this thread has made me want to vomit (not peoples' opinions, but just remembering some of those videos and envisioning having to shoot an animal), and I'm not even a vegetarian; I just really really love animals. Though other factors were into play to make me more sensitive during that time, I cried when I saw the chicken getting its leg broken in The Walking Dead; I'm that 'bad,' I guess. I'd definitely be a vegetarian if I could afford it and had the time to prepare vegetarian meals, but I do eat meat pretty sparingly as it is and can't eat anything that's been boiled alive or that I had seen alive previously... if I had to hunt for my food I'd probably be a vegetarian.
Either way, with that being said, I think the best thing that can be done for livestock or any animal used for food is that it be treated as humanely as possible before its death. I seriously become nauseated driving home from school and seeing hundreds of chickens cramped into a truck (while I live in a small city, it's close to agricultural areas and seeing farmers is not unusual) with crap all over themselves. I don't believe animals were never meant to be used for food or other things when we need it, but we need to show some respect for the fact that they have (though it's not very voluntary on their part) and cooping them up and not letting them have any semblance of a life before death is insanely cruel.
It should be pretty obvious that I could never see myself being a livestock farmer or a hunter unless we all went back to hunting/gathering... and even then, I'd prefer gathering, haha.