All animals feel pain - that's a given. It's built into...

All animals feel pain - that's a given. It's built into virtually every biological organism as a survival mechanism. The pertinent question isn't about pain but suffering. Only animals who have a self-image are capable of suffering. So whilst a crustacean may squeal and squirm when dropped into boiling water, it's unlikely to be suffering. Personally, I wouldn't kill a crab that way because it does seem cruel, even though it probably doesn't cause any suffering as such. Higher order animals like octopuses may have a self-image, so that's a different matter.

I think if I had to kill my own food I'd become a vegetarian pretty quickly. I'm one of those people who is happy to eat meat so long as someone else does the killing. I'm not very consistent in that regard.