I too would't try mixing these lines for a longer campaign. You have other options at hand, though.
First, being a villain doesn't necessarily mean acquiring corruption on a regular basis (well, not more than usual for an acolyte ). In fact, a memorable villain might be smart enough in his dealings with daemons to avoid that fate for a long time.
But let's assume that whatever the player/character wants to do brings corruption with it. You're probably best off with inventing ways to stave that effect off and handle them narratively, meaning there things the character has to do in order to keep him safe from corruption which on the other hand might get him in trouble with his other duties. Covering himself in hexagrammically warded clothing for most of the time, inflicting sacrificial wounds with a holy relic on a regular basis...analogous to the optional rules for removing insanity.
Though, if your player accepts that sooner or later, the character WILL go down in a blaze (monstrous, fiery or otherwise), there is a much cooler option. Dark Pact!
Think about it, it shelters you from corruption to a certain degree, if i recall correctly, because you're already "pledged" to a specific entitity, it gives the character a bunch of stuff to hide from his teammates...and he does get abilities that are very useful to their work, in the short run. So you could just portrait these as "miraculous", the character could claim to see saints when these things happen.
And then, at a point where it has maximum impact, there comes the pay day. Whatever the team is doing goes to hell, and all they might be able to achieve is to try and save their comrades soul while destroying his possessed body. Sound good?
Anyway, if you're already in the midst of it, let us now how it went .