Here is an opinion for you.
I was intrigued by the stranger at the end and although everyone else believes it was Karen Stone's "death wish" to submit to this stranger, or whatever, and that she will probably die at the hands of this stranger, I have to disagree. I perceived the ending differently. I feel Karen Stone ends up with this person because she wanted to be wanted. This guy obviously did want her. He had been virtually stalking her throughout the film. He had even shown that he was sexually aroused by her. She saw that in him and just wanted someone to want her and make her feel like a woman again. Yes, it is dangerous, yes it is naughty, but she was going to be wanted again. That's it.