I use pruning shears. It's one of the worst aspects of hatching. Even harder when you've watched a mother hen leave her nest with a sick chick struggling to survive and mum isn't interested. Yes, you could probably save some but then what. I tried it once and the mother just drove the chick away.
This is imo the best method.
Serious review: Thank you for this quick, humane and un-messy method of culling a hopelessly sick or injured chick.
It seems very effective and easy to remember.
Funny review: Also not nearly as cold blooded as doing it while having to look that little peep bunny in the eye while culling him.
Bonus points for your teaching us what color a peep bunny bleeds.
Although none of us want to go through a chick culling it is sometimes needed.
Clear instructions with clear photos of the process (even with a stand in for the chick). This should help take some of the edge off for first time cullers.
Well done.