Originally Posted by
qualia
Additionally, I'm not sure how you get that he "doesn't have willpower". It's true that the "having faith in his ability to use it" is more debatable (this is different from just "having faith", mind you). However, one can argue that after being shown that vision by Xe'ra, he must be a lot more confident in his ability to use it when the time comes. Why not? He's seen himself using it effectively, after all.
Lastly, becoming a vessel of the Light doesn't mean he'd stop being an anti-hero. This has been stated multiple times - the Light isn't picky. Illidan can remains the selfish (he is growing out of this, though), power-hungry and reckless anti-hero that he is, and still be able to become a champion of the Light as long as he fulfill the requirement (willpower or faith in his own ability to use the Light). Xe'ra pretty much said he was chosen because he is a good demon-killing machine, not because he was a shining beacon of justice.