When her mother (Rowena Ravenclaw) was on her deathbed, she asked Bloody Baron, who was badly in love with Helena to find her, so that Rowena could meet her daughter for one last time. But Helena refused to return with Baron, to which he got angry and murdered Helena(by stabbing her).

One may also ask, why did Helena Ravenclaw want the diadem destroyed? Helena Ravenclaw She stole her mothers diadem, in hope of using its power to make herself wiser, and fled Hogwarts. Her mother fell fatally ill and in spite of Helenas betrayal, wanted to see her daughter one last time. Rowena sent the Bloody Baron, a man who once loved Helena, to find her.

Just so, what did Helena Ravenclaw say to Harry?

Helena Ravenclaw: I stole the diadem. I sought to make myself cleverer, more important than my mother. I ran away with it.

How old was Helena Ravenclaw when she died?

Born in the year 982 she died sometime between the year 996 and 1000. She then became a The Ghost of Ravenclaw known as the Grey Lady.