My reading of Oddi's illumination is that he becomes intellectually convinced of the tenets of Illumination, but dislikes it and tries to reject it in his personal life, much like people who have been intellectually convinced of something they find unpleasant - like that we don't have free will or that life has no meaning - may still do their best to reject it and live their lives as if it exists. But at the same time, you can't go back to your old way of thinking - Oddi might be an enemy of Ralzakark and the Lunars, but he can't do it in the old, thoughtless, unconsidered way, and Illumination ought to really mess with your ability to hate and fear in the same way as you used to, because those are exactly the emotions it tries to do something about.
So Oddi wishes he had his old life back, and tries his best to work up his old feelings about Chaos and Lunars (but I repeat myself), but he can't quite do it any longer. It still doesn't stop him from pursuing and enacting a powerful plot against them.