I once had a job as a technician in the Psychology department of a university. The Experimental Psych course would introduce students to the Skinner Box and positive and negative learning reinforcement in rats. My job was to make sure the rats were on hand on schedule, survive the semester, and then I had to dispatch them at the end of the semester.
It was one part of my job I really, really hated.
I would put all the rats (like 30 of them) in a carrier and take them to a secluded lab and lock the door. We had a small surgical steel guillotine made to cleanly sever a rat's head. That was supposed to be a humane way of rat euthanasia. While in the cage, the rats were their usual selves, frolicking, climbing over each other and in a generally good mood.
I'd put the cage in a corner, take one rat at a time, push her little body into the chute so her head protruded out the hole and quickly pull the lever.
Chomp!
The head fell into the sink and the twitching body was summarily tossed into a trash bag.
No muss, no fuss. No noise. But I can tell you the truth...As soon as that first rat's head got chopped off, all the other rats in that cage just freaked out. There was panic in that cage.
Don't understand it, never will. But my wife will still attest to the fact that on those days when I came home from work, she could see it on me. "Had to kill the rats again, huh?"