His sister was the only person in the world he ever thought he would love if he was capable of it. She was the only person he cared about in any way whatsoever, he didn't even care about his son. He's a psychopath.

When she died he went off into the storm with her to die as well. It didn't work, so he ran away to live a more solitary life because there was nothing holding him to Miami any more.

Almost every scene you see him in in the movies he has a constant internal dialogue, his 'dark passenger.' That final scene where it's dead silent, where he isn't thinking about anything at all, shows that something happened to him. He might still have urges to kill, but something big changed in him when his sister died.

I thought it was a pretty good ending to the series.