Brutality + Envenom

The way Brutality is worded makes it sound like it should still function with Envenom. All chaos damage from my hit is negated, however, Poison is a debuff applied based on my physical damage. Why does Brutality seem to reduce my chance to poison down to zero?

The debuff is from the physical damage of the skill, brutality changes any damage coming from the skill. Since the debuff is from that skill, you will not get the chaos bonus from the debuff.

So, just to be clear, Brutality reduces chance to poison down to zero?

Yes. Because the game sees "poison" as part of the damage of the supported skill, and because poison is "chaos" type damage, brutality disallows poison from being applied.