Happy ending? Good ending!

Recently, I have seen a musical called Carrie. To me, it's not a familiar name but after the show, I cannot sweep this name out of my mind. This story is talking about a girl, Carrie, who got bullied by her mother and all of her classmates. There was another girl named Sue who felt regret and wanted to help her. Sue asked her boyfriend Tommy to ask Carrie to prom with him. This lovely couple became the queen and king at the prom. Suddenly, Carrie got wet by a bottle of pig blood. She went crazy and used her magic power killing everyone at the prom. When she got home, her mother thought she was devil. So, her mother used a knife to cut in her body. Of course Carrie killed her mother, but finally she also died.
In the middle of show, I didn't understand the story. I told my teacher I guess it will be a happy ending. However, when the ending was closer, I was shocked. It's totally a bad ending.
However, this ending taught me more things than if it was a happy ending.
Maybe it's a very bad ending, but to me, it's a good ending.