La Pianista - why did she break the girl's hand?

Question

In the middle of the movie, the main character (the piano teacher), breaks one of her students' hands by placing a bunch of broken glass in the pocket of her jacket, destroying her musical ambitions forever.

That she did this out of her jealousy of the girl's contact with her love interest, Walter, is obvious, but it's also a multifaceted action. From IMDB:

"She destroys the musical prospects of an insecure but talented girl, Anna Schober, driven by her jealousy of the girl's contact with Walter -- and also, perhaps, by her fears that Anna's life will mirror her own."

I also believed at the time that she was disappointed by her student's failure, and so she broke her hand to destroy her musical ambitions. I saw this as part of her icy, dictatorial, perfectionist personality.

Could someone explain the motivations of this action in sufficient detail?

Answer 1

I am just going to repeat the lines for the consistency of the answer:

From IMDB, the scene is described as:

Erika destroys the musical prospects of an insecure but talented girl, Anna Schober, driven by her jealousy of the girl's contact with Walter — and also, perhaps, by her fears that Anna's life will mirror her own.

This is the only reason why Erika broke Anna's hand. From the start of the movie, we saw how Erika had lived her life, and we saw the psychology of a "sexually repressed" woman. She wanted a partner in her mid 40s to fulfill her dream of a fetish relationship. In the meantime Walter emerges into the scene. She was attracted to his charming nature and she could not resist herself from thinking of Walter. Although she tried to let herself understand, this is not right, but she barely could stand the attraction in her mind. So when she saw Anna with Walter, she became envious and that is why(only) she planned to break her hand.

But I am not convinced by the fact that she did it because she was a perfectionist and she was not happy with the performance. If Erika had such a personality, she would have spoiled many students' lives until then. Those accidents in turn would harm the reputation of her class. Parents would be frightened to send their children there and no teacher wants such a thing to happen. So I think only the romantic activity with Walter led Anna's hand to be broken by Erika.