Sushi, the first few times I had to cull new hatched chicks I got a little zip lock bag, put a napkin in it, held the baby firmly with face pointing down away from you, for just a minute. Then with scissors snipped the little head off or break the neck with the scissors. Don't worry if you don't cut through the neck, but I do make sure it does bleed.
It may shiver in your hand, that's OK. It is not suffering, it is the nerves and muscles reacting to the lack of oxygen and blood circulating. All deaths entail this, all deaths, it is a fact of life and death. Take comefort in that you held it as it died.
Close the bag and I put it in the freezer until trash day. It's done, the baby doesn't know what was going on. The only one suffering is you. You will get over it. If you hatch any type of eggs, it is sometimes necessary. If you raise any animal it is sometimes necessary when they are older. It doesn't really get easier, but it gets easier to handle.
I still do it this way and I have tried the can with Vinegar and Baking soda, which I imagine is like the ether.