Why is murder wrong if Many Worlds Interpretation of Quantum Mechanics is true?

[FlatAssembler]

Fish do feel pain, which is why they're anesthetized and/or stunned (among other things) before the kill in operations with an ethical focus.

I think the scientific consensus is that they don't. And the arguments for that are fairly easy to understand:

They do not behave as if they felt pain: a fish with a hole in its fin continues swimming normally, rather than trying not to use that fin. Also, fish do not have the neuroanatomy necessary to feel pain: they have little or no type-C neurofibres, which are necessary to feel pain (we know that because people with congenital insensitivity to pain have fewer type-C neurofibres), not to mention the brain structures.

If fish can suffer (be it only physical pain), then the right thing to do would be to stop eating them.