I did my first this week. Slit the neck and it was screeching. I couldn't bear it. I cut the rest of the way through the neck. I had to saw! It was awful. The body thrashed so hard that it pulled the cone off it's screws and the body flapped/ran 20 feet away before dropping dead. I realized the severed head was still in my hand . It was blinking and the mouth was gaping open and shut. Total nightmare and I still have six more grown out.
Im sorry for your experiences. It must have been awful. I hope you can find a better method.