I have a theory or I guess headcanon technically that a lot of the things Erik does aren’t planned.
Take for example, the mirror scene. I don’t think Erik planned on leading Christine to his home that night- it’s too on the spot. He doesn’t even let her change, and I feel like if Erik had planned on taking Christine to his home that he would have planned out a much more “mysterious and sensual” kind of thing.
But the reason he takes her down is because Raoul. All of a sudden there’s a threat, a major wrench in his plan. I think Erik panics, and that’s why he makes the quick decision that Christine must come down with him that very second.
And then another example is his proposal on stage. I think that whole thing is planned, but what wasn’t was Christine unmasking him in front of the entire audience. I don’t think Erik was counting on her saying yes for sure, maybe he was, but I know he wasn’t expecting his face to be gaped at by a packed audience.
And I think he has an awful moment then. All the terrified, disgusted faces staring at him probably sends him into a horrible panic. We know Erik has a troubled past, and we know he was displayed at a freak show, consensual or not depends on the version. Even if he did willingly let people pay to stare at his face, I’m sure he didn’t love it and probably feels ashamed when he thinks about it.
So now he’s on stage, pleading with Christine to marry him, and his mask is off for all the world to see. I think this is what makes him snap and what leads to Christine being kidnapped from the stage. Because Erik literally panics and tries to escape from the people staring.
Not an excuse for his actions of course. Erik doesn’t get a get out of jail free card just because of his awful past, but I think it’s interesting to consider why people do the things they do. Just an observation and idea of mine.