Its hard to discuss this particular book because it evokes so many different feelings...

I read your ending and thought you did a nice job of capturing the
way I felt about Stephen.

O, like Beauty, fell in love with her submission.
Beauty fell in love with each master for what
they could give her and teach her. She was a cherished slave
and eventually discovers herself and what she wants/needs and
is willing to pursue it.

But unlike Anne Rice's Beauty, O's relationship with her masters, including Stephen, was especially abusive. She is permanently marked - branded - and as Mobius points out she is allowed to end her life. If you read the other alternative ending, then she never regains her freedom.

With a healthy relationship, submission is a form of its own power.
I often wondered what power O had besides submitting. She didn't
seem to ever say "no".

* Steps away and prepares for the flaming arrows to arrive *

For those of you who haven't read the tale, it is available in text format free on many web sites.