Just a thought I had, the 40k lore has plenty of characters that turn to chaos, but does it also have characters that somehow turn their backs on chaos and free themself from its corrupting influence?
There's an entire chapter of renegades called the Soul Drinkers that manage to do that, but chaos mutations are permanent an the taint is permanent. Risk of relapse (which for them is high as well) as well as the mere knowledge of chaos being a sketchy subject in the Imperium means there are no second chances for those who have turned to chaos (at least, short of those who take suicide missions. I do recall one story where an Imperial Guardsman from a penal legion gets possessed and in a moment of clarity induced by a psyker null, decides to launch a suicide attack to do the Imperium one last service).
Then there are the peoples that manage to live in Chaos worlds/Chaos influence/Chaos occupied worlds and not bear chaos taint at all, like the Partisans in Gaunts Ghosts or the Cadians who have limited (potentially warp related) mutations that cause their eyes to be pink. Admittedly the latter have loads of heretics and mutants anyways, and the former may have been sheltered due to their intense faith, remote location and ritual use of poison lotions.
So in short, there are people who turn their backs on Chaos, but unless you find a rogue inquisitor willing to accept you, the Imperium will sooner shoot you if it discovers your previous affiliation than accept you. That sounds like a cool story idea actually, a reformed heretic trying to escape notice from his chaos tats, or try to avoid cultists bringing him back into service.
There is of course, always a third way out. There's a really neat short story where a 40k governor is doing normal governor things.
Lazing about, living in luxury, doing whatever to pass the time - just making sure the Imperial tithes get sent in time. He's not too pleased with the Imperium and he is only grudgingly tolerant of their presence, not even being a fan of Empy.
Further down the line his incompetence results in Tzeentchi, Nurgly and Slaaneshi demons exploding forth from various parts of the planet. The governor gets so angry that it opens up a Khornate rift, completing the Chaos takeover with his one last act of incompetence.
Then in a moment that for just that moment scares even the Bloodletters (unsure as to why the governor was no longer scared) that just appeared in his office, he realizes he can tell both Chaos and the Emperor to fuck off - shooting himself in the head.
So yeah the third way out is suicide
grimdark