I'm tired of seeing the word grooming taken out and used all the time by people who probably don't understand its definition.
For the record, Christine was in her twenties, when she met the Phantom / Erik. She was a fucking adult, in a society that for the time considered her a spinster. She never turned 15 or 16 in the original story, as people are so fond of claiming.
Also, Erik is not a villain but an anti-hero character. I'm tired of seeing him constantly demonize, like he's just a bad guy, when he doesn't.
Erik and Christine are basically twisted versions of Beauty and the Beast, Hades and Persephone, and Romeo and Juliet. Without forgetting that they are also, among other things, the incarnation of death and the maiden.
Erik also symbolizes Christine's sexual awakening and coming of age.
Whether you like it or not, they have a romantic and sexual connection in the original text.
Didn't Christine got adopted by the Girys when she was something like 8 ? So even if they meet when she's 20-22 in the musical the Phantom has been her teacher for 12-14 years, using what her father told her about the angel of music.
But again, the Phantom is not meant to be a good guy, he's just very angry to lose Christine when he gave her all his knowledge and she ran because SOMEHOW killing people is not good for a relationship. But from a guy who lived in the sewers for the last 20 years it's understandable.