If Sulfin Evend knows who and what Lysaer really is (from Peril's Gate), in Traitor's Knot why does he persist in the belief that it was Lysaer who saved him from the Grimward, even after Asandir tells him the truth? He's seen so much - he knows of the corruption of Lysaer's cause, of the whole Alliance. Why won't he listen to Asandir, when everything else he believed about Lysaer has been proven false?
Now if you would like Janny to answer your question you will need to indicate you would like the author herself to give an insight.
As I see it, at the time Asandir talked to Sulfin Evend, he was having trouble just getting the Alliance Commander through the door of Althain Tower he was that distrustful of Fellowship Sorceror's. No way was Sulfin's faith in Lysaer going to be shaken by anything Asandir said.
He had seen enough to know Lysaer was flawed, and he had seen how Lysaer suffered for that madness he could not control. Sulfin Evend still believed him to be the blessed Prince, guiding light of the Alliance and still thought Lysaer had saved him. He had proof of Lysaer's commitment and gift of light and the circumstances of his 'saving' lent strong reason for his belief Lysaer had saved him from the darkness. He had chosen not only to give his allegiance and friendship, but also because he thought he owed Lysaer a life debt, had also sworn an oath to the land and was prepared to repeat that oath in front of a Fellowship Sorcerer. He had given his word and although he was terrified of the Sorcerer's and distrusted them he was going to see it thought to ensure he could protect Lysaer. His thinking there was not blinded, Lysaer under power of the necromancers would have been a disaster, Sulfin Evend was willing to pay any price to prevent that, which is how Enithen Tuer got him to swear an oath to the land in the first place. There would be something in that which has not come to light yet.
Even at the start of Stormed Fortress we can see Sulfin Evend despite his strengths is still not seeing the past that clearly. His own misconceptions colour how he sees things. He was was still loyal to Lysaer at the end of Stormed Fortress, if ever a reason came up for him to forsake that loyalty, we have yet to see it. Enithen Tuer's comments in Traitor's Knot seemed to indicate he would one day have to chose which oath to forsake.
'You are a loyal man, Sulfin Evend. There lies your strength and your downfall.'
One day fate will force you to choose which of two loyalties you will sacrifice. The land does not bear a blood-sworn oath lightly. The powers you invoke will be greater than you, and they will not treat with duplicity.