I could definitely see the Revenant Legion becoming the first truly "Traitor" legion - no Sanguinius, they have to keep on as they have been going, they become more and more isolated from the Imperium and then they realize that they can "save" Mutants. The Imperium doesn't need to kill ALL mutants - they can become the stock for the Revenant Legion. So they intercede on some Compliance mission where another Legion is genociding a Mutant human population (as you do when you're an Astartes in the Imperium of Man), coming to put a stop to that Compliance. Which, unfortunately, leads to combat between the Legions, and the Revenants realize that they've crossed the line of no return - if they want to save the Mutants from the Imperium, they need to know what the Imperium knows. And start to eat the bodies of dead Astartes from other Legions. Little bit of Alpha Legion brains, little bit of Night Lord brains, add in a dash of paprika and the brain of a single Word Bearer, and who needs Chaos to destroy the Imperium?
*applies holy oils to the algorithm* Spirit of the machine, heethe my will and let this small warhammer creator gain more audience to spread holy knowledge
