Of course he was angry he just watched his boss die in front of him. One who at first wanted to kill Alleria but was talked out of it by him and Lothraxion. Clearly up until that point he had no reason to doubt the light or that light beings could want different things. Also Velen explains to him a ton of shit that he was never told by Xe'ra like that Naruu can go void.
Why would he justify it? Even in the vision of Stormwind he didn't go light Hitler. He tried saving Alleria. It would be incredibly bad writing to say that even in the worst timeline he stays good and yet decides to go evil after finding out Xe'ra lied to him and kept him in the dark about alot of shit.