I want to kill a fish
Did you have a need to eat fish for lunch today?
No it's because I don't like those fish
If you dislike something it seems you think it's okay to kill it, but that is barbaric. I would command you to just let it be and live, instead. Every living being has a right to live, so be more humane unless you are starving and need it to live.
I think I'm still going to kill it. I don't like it.
Killing an animal for no good reason is really cruel. I could see if the animal was dangerous, but you shouldn't kill the fish just because you don't like it.