Actually, I have read a story about an Ultramarine purging some Chaos world who becomes stuck under some rubble and then a Cultist comes up to him and is like "Dude, why all the RIGHTEOUS FURY?", to which Ultramarine reply is obvious, claiming that the guy is "WRETCHED TRAITOR HERETIC MUTATION DIE DIE DIE", but then the Cultist goes "Dude, but I got born here. I never had a chance to even see an Aquilla before I was 14 and could handle an autogun. We're living relatively peacefully here minding our own stuff and try to not get sacrificed or raped by deamons and then you burst down the door and murder us all. What the fuck, dude?"
Then the Ultramarine gets up and mushes him into paste with his Power-Fist.
Nevertheless, it's worth noting that a lot of the Chaos guys actually didin't have a choice. To actually, truly and willingly fall to Chaos you need more than just "JOIN SLANNESH WE HAVE COCAINE COOKIES", at least usually, most of the people are mind-controlled by Warp and so on. One of best examples would be Fulgrim - the blade he had was slowly corrupting him and he wasn't completly aware of his actions, and when he finally woke up (after he killed Ferrus Manus, his best Primarch bro) from it, he was utterly crushed by what he had done, so he went into Villanious BSOD and let himself get locked in a painting while his body got completly overtook by the Deamon.
He got better (or rather, worse), though, and after thinking a lot about stuff he decided that joining Chaos is not that bad after all and kicked the Deamon out of his body.
The other example could be Horus (yeah, I know it's mostly Primarches but Warp damn it, when was the last time you had a regular Chaos cultist POV that included him before and after joining Chaos? Exactly...) - it is said that in the last fractions of seconds as he was being removed from existence by Emperor the Chaos left him and he was again himself.
Of course, this doesn't really change anything - the only effective remedy for CHAOS is death, so don't worry and PURGE THE HERETICS.