1. Always follow the fishing regulations.
2. Be respectful of others.
3. Treat the resource with respect
I follow those rules. I keep a fish or two when backpacking and bonk the heck out of pacific salmon and dollies in Alaska. But generally I don't kill fish because I just enjoy catching them again.
I don't think it's right to tell another person they can't keep a fish as long as the regulations allow it.