I don't understand why milk is avoided. You are not necessarily harming the animal.

It's about reducing harm imho. So if you can reduce overall harm by cutting one of your friend's arm, do that.

Imo harm does not equal to kill. A fish with limited imagination cannot be harmed if his life just ends and he does not feel anything.

You are right, but I believe putting a cease to life is not inherently bad. If we could kill animals without letting them feel anything, that wouldn't really be bad.