Well, decapitation is relatively humane, it doesn't destroy commonly used meats, and helps drain the carcass. Or at least it is about as humane as you can get in small scale operation.
If you are going to eat something you are going to have to kill it. You can't use chemicals to put it to sleep and die, that renders the meat poisonous. Same thing for painkillers. For larger animals you could use something like
this but not really for chickens.
Much of the shock I usually see stems from the fact that people are so far divorced from the production of what they eat these days that the realities of animal husbandry are a rude awakening, particularly the slaughterhouse. Violence of any form(even on American TV/Movies) is so sanitized that watching an actual death, even be it a chicken, isn't something most people have to deal with. It's ugly, dirty, messy, and it goes on much longer than you have been lead to believe. It's not a fun thing for any balanced individual to watch.
People also tend to anthropomorphize their food animals some and assume that what would make a person happy is what makes a pig happy. True to an extent, but less than you probably think. Those who raise animals for food have a vested economic interest in ensuring the animal is not ill-treated, even during slaughter. Stressed animals are less healthy, prone to disease, reproduce less effectively, and yield lower quality meat.
Probably the best thing we can do if we are truly interested in effectively and ethically consuming our food is to better ensure we are less
wasteful than we are now. If we are going to grow it and we are going to kill it, it seems the least we can do is make sure we make the best use of it.