Easy.
Let's say you're a crewman on a C-130 dumping Agent Orange on a night mission. The pilot gives the signal, and you let fly with the stuff.
Whoever set the mission planned it for an area with some jungle, some rice paddies and some villages. The Agent Orange is spread over all of them.
While Orange is technically a herbicide, it's hard on all living things. Spraying the rice paddies and villages was using a WMD against civilians and the food supply.
A war crime.
Would you say the airman was a war criminal?
Or were the pilots who firebombed Japanese and German civilians war criminals?
Arf