I agree on the reductionist part, it's never that simple. But, in his case it was his conscious decision to use the gas as a chemical weapon, the case of which was unprecedented. Sure anyone else would have come up with the idea sooner or later, but he has set the precedent so that anyone after him now can use it, as the Allies actually did, in all this "but they also did it, now we want too!" sentiment. His bad judgement caused not only good things, but disregarding the possible consequences also cause much much worse things. It's actually simple, imho.