I don’t understand the point you’re trying to make above.

In this case specifically, the outcome isn’t unclear. Let’s call the crab’s pain one unit of pain, and that unit can directly alleviate 20 units of pain across a handful of other beings. The utilitarian ought to prefer avoiding 19 units of net pain, than having 19 units of net pain occur.

I read your initial post to be some sort of utilitarian moral argument, roughly, that less pain is better. Or something like that. That argument, in this case in particular, leads in the opposite direction than I think you want.