Well, first you have to kill it.
It was sickening to watch this gif. I turned away, winced and made a gagging sound as I watched. It takes a lot to make me puke, so it didn’t make me nauseous.
This is one of the ways you kill a crab. First you freeze it in a freezer. Then you take it out and chop its head off. As it’s immobilized and unconscious from being frozen, it doesn’t feel anything.
You can always drop it in boiling water. A lot of people say that this is cruel because lobsters and crabs scream when you throw them in.
This is not true. First of all, these creatures do not have lungs, so they cannot scream. Second of all, the boiling water kills them so fast that they don’t have time to feel anything. Lobsters do not even have a real brain (how is that possible!?) and crabs can’t be far ahead of lobsters. So I doubt if it suffers much in any case.
The “screaming” you hear is not screaming. First of all, the thing dies as soon as you throw it in the water. The “screaming” is simply the sound of fluid escaping from the creature as it is boiled. The fluid leaves the body rapidly, producing a squealing sound. There’s nothing to worry about.
All the same, I think I would rather drop a crab into boiling water than saw its damned head off. Gross!