Excellent, clear and incisive. This is along the lines of what I've been thinking about it. It's more about what they perceive than about what we think or feel about it. And we have a hard time guessing their perceptions; all we can do is judge reactions to actual events.
Great comment, thanks.
Exactly. And, if I may go on a tangent again, in my experience, I've found I absolutely cannot predict their reaction to situation by looking at their reaction to a different one. Like I said, animals have spotty event perception and where those spots occur is sometimes entirely random-seeming. The capacity for something in one situation does not in any way mean it's going to be applied in another. They can be both prey animals whose entire lives revolve around staying alive (they are) and opportunistic little vultures that line up to watch you gut their flock mate for the chance to steal a few scraps. It's not a contradiction, it's just perception. While birds in a flock do look out for each other, by and large, it seems a selfish act. I do not mean that with the negative emotional connotations it often has. I mean they're interested in personally surviving, in personally passing on their genetics; it doesn't seem to come from a place of altruistic empathy for the other birds like it does for humans. Again, I am AM NOT saying they can't form bonds with other birds. I am not saying they're purely instinct driven robots. I'm saying that the experience I have with this situation points to there being a spot here, particularly in larger flocks. Whilst chickens can recognize many, many individual birds, the importance attached to the individual seems to go down rapidly after, hmm, about five? If you butchered one bird out of a pair or a trio in front of the other/s, I'd not at all be surprised if there were a different reaction. Their flock would suddenly be massively different. If I catch a hen from the main flock, none of the roosters bat an eye. If I catch a hen from a breeding pen with only a few birds in it, the rooster freaks.
Properly restrained death throes won't cause alarm, as it sounds like wiggling noise, not flapping. Chickens don't seem to react to the smell of blood either, not like horses will. Chickens are alarmed by sudden, sharp movements, loud noises, and aggressive body language, among other things. I do my best to keep the entire process calm for their sake, my sake, and the sake of the bird being butchered. By the time they know something's different from any other time I pick them up and hold them, they're unconscious or nearly so. It's not like a hawk attack where the unfortunate victim shrieks in distress and flaps around very vigorously and for much time before being silenced. All of that is a clear warning to the live birds that something's wrong and they could definitely die too, so they clear out.
I still need to figure out how to edit this to make it clearer, but I wanted to get it posted before going out to feed the chickens, so if one intends to quote this post to tell me how wrong I am I'd appreciate if you waited until I've refined the language to what I actually mean.

I realize I've said a lot of contradictory things in here to the opinions a lot of others have said, but it truly is not meant to step on toes, only to share some of my reasoning behind earlier posts.