The Devastator of Victory Bay
Before his induction into the chapter, Avitus grew up in a small settlement on Typhon Primaris that was terrorized by a corrupt regiment of the Imperial Guard. The settlement was extorted to near starvation by unofficial taxes and penalties, and often served as a target of the Guardsmen's wanton cruelty.
While fighting on Kronus with Captain Davian Thule, Avitus's hatred spiked to new heights during the battle of Victory Bay, against the rebellious 1st Kronus Regiment. Cracking the Guardsmen's defenses was a savage and bloody affair, which cost the lives of two of Avitus's squadmates. Avitus has since seen all Guardsmen as traitors and weaklings.
In Warhammer 40,000: Dawn of War II - Chaos Rising, one of the five primary supporting characters - Sergeants Tarkus, Avitus, Cyrus, or Thaddeus, or Librarian Jonah Orion - may turn out to be the traitor feeding information to the Black Legion, depending on the player's handling of the single-player campaign, and the allocation of "corruption" among these characters. If these characters reach the crucial point in the campaign with equal levels of corruption, the traitor is identified as Techmarine Martellus.
In Warhammer 40,000: Dawn of War II - Retribution, Tarkus states that the traitor was one he fought alongside at Kronus, and that they became regular friends during the Tyranid invasion. This "official" traitor is never named, but this description seems to narrow the possibilities to Avitus or Thaddeus, since Cyrus, and Martellus are both present and fighting alongside Tarkus during the events of Retribution. Of these two, only Avitus definitely participated in the Kronus campaign, and Thaddeus at one point asks Tarkus if he fought the Necrons on Kronus, which suggests he was not there.