Nobody is saying that fish are moral agents that can empathise with other beings. That doesn't man that they're not moral subjects; the ability to understand that one is causing harm is not a prerequisite for the ability to suffer oneself. I think everyone knows this intuitively, but it does feel good to have our less moral habits be justified by memes that we would otherwise find to be illogical.
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.
Ethical consideration has to extend to more than just painless death to be worth a damn. I can't walk into an infant ward and painlessly murder infants in their sleep for a reason.
Yes, the reason being they are human.
Why is that a reason?