What makes "The Birthday Party" so unsettling?

I think the story is sad and abusive. Lulu looks to overcome her loneliness, first by trying to interest Stanley, then, at the birthday party, by flirting with the much older Goldberg. In the aftermath of the party, he goes to her room and introduces her to some sort of deviant sexual practices, aided by unidentified toys and devices carried in a mysterious briefcase. In the last act, she claims that she has been abused and abandoned by Goldberg, who dismisses her with the suggestion that she got exactly what she wanted.