Why Christine removed the Mask?
Of course, the recent movie in 2004 and the Musical have different presentations of it. In the movie, recall Raul talking with the Managers about a plan to capture him. They would act out his Opera, and Christine would betray him somehow, but they don't say exactly how. My guess is that removal of the Mask was the way - the ultimate betrayal. She sings about being torn, not wanting to betray the man who once inspired her voice, etc. But, Raul and the others persist and she agrees to do it. She seemed to be pretending that she was under his influence (as she had been in past scenes where he appears to hypnotize her.) At the right moment - when he is vulnerable she takes the action. In the movie, there is a delay before he exits with her. In that time, he looks into her eyes - if you look at the expression, it appears as though he is hurt or shocked that she did this. The movie made more of a tragic love story out of the story. In the movie, I believe she loved both men, just differently. I don't think she recognized that right away though. As for the musical - there I am not certain. I saw that in the 1980's and at that time only 1/2 understood.
The original book was more a horror, not so much a romance. In 1911 when it was published in the US in English, some information was probably lost in the translation. I don't recall if there was a time in the actual book that she removed his mask for any particular reason.