I should note I'm speaking of the movie more than the musical as I have never seen it.
I was watching Phantom of the Opera recently (For something like the 40th time XD I stopped counting long ago) and I was pondering Christine's mouthed "I love you" to Raoul and the final kiss when something came together for me. I noticed, as she waded into the water singing, "...God give me courage to show you, you are not alone." she slipped on the engagement ring just before she kissed him. As I had understood the situation before, it was just a way she was begging the Phantom to let Raoul go. The mouthed "I love you" seemed to be a betrayal to the Phantom, her reassurance to Raoul that her kiss with the Phantom meant nothing. But if she had agreed to marry him, that changed things. It made the I Love You more of an apology to Raoul, who had been begging her so earnestly to refuse the Phantom (Who I'll refer to as Erik henceforward, even though his name is not mentioned in the movie/musical and I like to keep the musical and book seperate as in a way they tell different stories) and the kiss, I suppose, of her way of commiting herself. You could say her words, 'Pitiful creature of darkness, what kind of life have you known? God give me courage to show you, you are not alone.' were her way of convincing herself she was not making the wrong decision. We've accused her of betraying Erik, of being unsympathetic to his lonliness and pain, but she had shown with that decision that she was willing to, as they say in the Harry Potter universe, choose what is right over what is easy. She may have wanted to stay with Raoul, but choosing to stay with Erik, in my opinion, is the more moral decision.
So, why did Erik let them go? She agreed to be his wife- isn't that what he wanted? Erik has been treated badly his entire life, abused because of his deformity. He even calls himself the 'Angel in Hell' and his pleading to Christine in 'Stranger Than You Dreamt It' after she takes off his mask the first time shows what a lack of self-esteem he has, he believes himself some sort of demon because of his appearance, as has been beaten into his head all his life. He's never really been taught to be tolerant or kind. He is gentle and affectionate with Christine, yet he is even rough with her during 'Down Once More', practically dragging her down the passages as she struggles to get out of his grasp. Joseph Buquet mocked him, he had no reason to care about his well being, he had been taught no morals that would keep him from killing him, so he did. But at the end, Christine decided to stay with him, and kissed him (Thoroughly enough :P), willing to be symathetic to him and help him despite the fact he had been so cruel to her and Raoul and though she seems to have affection and sympathy for him as her mentor and teacher, she does not love him as he does her. This is exactly the kind of sympathy and affection Erik has never been shown or taught in his childhood, that makes him such a guiltless killer. And I like to think he learns something then, showing Christine the love she has shown him by making the hard decision and letting her go with Raoul.
Though he still seems to think himself unworthy of her then, calling himself again the 'Angel in Hell', Christine shows again the beauty of her kindness and morality. When she and Raoul could've just left right then as Erik said, she goes back. She gives back the engagement ring, but I think that action in itself does not mean much. I think Christine wanted to show him that the kindness she had shown him earlier when she decided to marry him was not some way of putting him on- that though she does not love him, she cares about him, which is what her last look seems to say. Thinking about it now, actually, when he sings his last line to Christine, 'Christine I love you', maybe it means more than we thought at first. Perhaps he does not mean love as in romantically, but love as a person who cares for him, as no one (Besides Mme. Giry) has really done before. I think that says a lot about the line, 'You alone can make my soul take flight', as opposed to the 'You alone can make my song take flight' he says in Music of the Night. (That is what he says, yes? I'm tired, its almost 3:30 am >.>)
Really, I am satisfied with the ending, now that I understand it more than I did before. As for Point of No Return and why exactly Christine took off the Phantom's mask the second time, I think she was putting him on in PONR. She does not feel for him in the sexual way expressed there in Don Juan, and notice she seems rather reluctant to play her part until she looks up at Raoul who signals to the police, sort of instilling a confidence in her. PONR seems to mirror Music of the Night in many ways, except at the end of Point of No Return Christine no longer seems to be in a sort of trance, as Erik sings, 'Anywhere you go let me go too' she looks at him with a sort of sympathy, not affection. ALW says in the companion (I hear, I don't have it myself) that the unmasking was done "in the moment", I do agree with this, that she was not thinking of the consequence, otherwise she must've known he would take her as she told Raoul after she did that, but I think her intention was still to help in his capture, not to give him a chance to escape, ect. as some have predicted. I think that like the book (Gah, bad paralell, I know, I know), Christine does not really treat the situation with thought and maturity until the end, as like her book counterpart Christine seems hateful toward Erik ('The tears I might have shed for your dark fate, grow cold and turn to tears of hate!') up to the end, when she is faced blatantly with the decision between the life of her lover and her freedom, she really reaches a level of maturity and understands Erik more than she had back on stage in Don Juan and makes the mature and moral decision.
In the end, I agree with Gaston Leroux in the Epilogue of The Phantom of the Opera when he says, "He had a heart that could have held the empire of the world; and, in the end, he had to content himself with a cellar. Ah, yes, we must pity the Opera ghost.", because though the Erik he speaks of is in a way a different man, one element stayed true in the book's adaption to stage and then to the big screen- all Erik ever wanted was to love and be loved, but up until he met Christine Daae he did not know how. God Bless her, however ordinary a person she was for most of the book and musical, in the end she made a choice and took a leap more distinguished people before her hadn't dared, to love the feared and despised Phantom of the Opera.