I forgot to answer your question. Least traumatic for you or for the chick?And if I should cull what way would be the least traumatic?
My general answer is however you can. For the chick if it is quick and sure it doesn't matter. You do not want to hesitate or flinch. You could injure the chick instead of killing it or you could injure yourself. You don't want either of those.
You could slit the throat and let it bleed out. I don't do that but many people use that method when they butcher chickens. If done properly they think it is a peaceful death. You can remove the head. It is kind of small for the hatchet and stump method but that could work if you are sure of yourself. One method for chicks is to cut off the head. You can use poultry shears, pruning shears, pruning loppers, or even scissors. If you take the head off cleanly you cut the nerves so it feels no pain. It may kick and such but that's just nerves firing with no connection to the brain.
Frankly, if you could immediately crush it the chick would feel no pain but that would be traumatic to me.