In modern times, we have things like international law, the geneva convention, the US constitution, and other rules/structures guarding against chemical and biological attacks, cruel and inhumane punishment, etc.
In the context of the world of the anime, it's clear they don't have organizations like that, similar rules, etc.
They are just getting to the point where they are thinking about teaching and training within the Guild. Which is pretty rudimentary as far as things go.
Why would Priest and Elf girl try to prevent Goblin Slayer from using fire, water, or poison to kill Goblins (in ep 6)? Especially given that they've both been traumatized by Goblins.