It never ceases to amaze me how fundamentally people misunderstand war crimes, especially relating to bio and chem weapons. On one level they cause needless suffering and are really not that effective compared to more conventional arms, they are mostly terror weapons. On a more important note they tend to cause incredible collateral damage to civilians.
While ages past were brutal they still had some codes regarding war in most areas of the world, for example a town under siege that gives up in reasonsble time could expect better treatment, but if the attacker had to storm the walls then all bets were off and said town could expect a good sacking. Doing the latter to a town that did the former would be frowned upon in a lot of places in a lot of periods. Point is while ages past might not have had the Geneva convention, most "civilized" societies still tended to have some codes and norms about this.
Ultimately these "laws" are important because otherwise we get a race to the bottom, see WWI. Posioning drinking water to make people sick, especially whencivvies can be affected is a bad idea because what if both sides start escalating? Misery ensues, as our own history proves plenty. Most people that even casually study history should understand this.