Why does Williams chose to have an abuser win at the end?

I don't know if "win" is the right word. Stanley's life isn't going to get any better because Blanche is dragged off. Stella knows in her heart what has happened and one can wonder what happens to their relationship. I think, in the end, Williams is trying to point out a sense of being rather than a morality tale. Stanley represents a very unrefined manhood, a Romantic idea of man untouched by civilization and its effeminizing influences. His appeal is clear: Stella cannot resist him, and even Blanche, though repulsed, is on some level drawn to him. Stanley's unrefined nature also includes a terrifying amorality.