According to that picture book he isn't ignorant anymore. However, he made the decision a long time ago by killing his father, and now he just decides to see that road to the very end. It's also a kind of natural decision because their world is in a chaos and Suzaku especially isn't the kind of person who would bear to do nothing in such a situation. And having his love interest (no matter how unrealistic) killed probably pissed him off.

Lelouch and Suzaku became pretty much the same. Suzaku fought for the evil-doers to justify his own decision, and Lelouch abandoned to death and destruction his own forces - and country as well as his ideal to crush Britannia - just to go and save his little sister. While that's admirable on personal level, he practically speaking did what Suzaku did: Betrayed his own.