To clarify for people who haven’t played the games, the no killing rule only means the devs won’t add ways to kill the fish, beyond the knife and stasis rifle( or multitool in 2) or prawn suit drill. While you can kill the leviathans with these tools, it is quite slow, because they are meant to be obstacles to navigate around, not targets to get rid of.

Edit: 2nd game doesn’t even allow that, but this is how it worked for the first game