I am part of the squeamish camp when it comes to killing my own food. Yes it's partly irrational but I can't help it. So I've been reading up on the best way to kill a crab without seeing it thrash wildly as if in the hands of Jeffery Dahmer.
There's been a few posts here offering great suggestions. Yesterday, before buying a couple of live Dungeness crabs at the market, I looked up and read some websites about crab anatomy. Combining what I learned of their anatomy with past suggestions, I think I've got a pretty good method for the squeamish.
First the anatomy: Crabs don't have a brain, but they have 2 nerve bundles (called ganglia). One nerve bundle is in the middle, close to the eyes, just under the carapace. The other nerve bundle is also in the middle, but some distance from the eyes, close to the the softer whitish shell. I figured if I slice into one and maybe both nerve centers, I'll kill it instantly.
So here's what I did: I put the live crabs (still in bag) in the freezer for 10 minutes. When I took them out, I laid it carapace side down. It was not moving. I put my knife about an inch or 2 from the eyes (see photo) and sliced down hard, cutting towards the eyes. 2 or 3 legs jerked once, then it was motionless. I prodded it a couple times and it was still motionless.
At that point, I think I could've pulled off the carapace and continued slicing without any thrashing, for wok-fried crab, but I was lazy and hungry last night so I simply boiled for about 12 minutes.
We ate the crabs with a ginger-nuoc cham dipping sauce (similar to nuoc cham for spring rolls, but with grated ginger instead of minced garlic).