May I gently suggest that if one is going to own animals (chickens especially), it is important to come to terms with this issue so that you are prepared to do what's necessary. It may involve a change of mindset if you feel that only a bad person would Do That. If you ask me, it takes a GOOD and STRONG person to end an animal's suffering when it's needed.
I think it's our responsibility to BECOME the kind of person who can take an axe, or shovel blade, or whatever, and end a chicken's suffering when needed. It's part of the responsibility of keeping animals. None of this "I just can't". Piffle. People can do all sorts of things when the need is sufficient. You just have to understand that sometimes, you have to do this.
If you are concerned about being so nervous that you miss, wrap the chicken in a towel or etc so it isn't flapping or thrashing. If you are really concerned, get a square-bladed shovel, perch it on one corner of the blade next to the chicken's neck, align it so it will cross the neck at the right place, then stomp down hard on the shovel with your foot. This is what I've done on various poor critters that barn cats have drug in, mortally injured but not apparently interesting enough for the cat to finish killing, and I assure you, you can't really miss.
It's not fun; but some parts of life are not fun yet they're still the right thing to do.
Best wishes,
Pat