One key element in almost all revenge movies is that revenge doesn't work. It doesn't lead to happiness, it doesn't make you feel better, and half the time, the person seeking revenge dies. In this movie though, Driver gets his revenge, dumps his brother's ashes, and just rides off into the sunset. It's pretty much "revenge is good, I did what I wanted to do, and I'm happy now." It's just a really empty ending. I get the feeling the writer was like, "Well, Driver is too cool to kill, and "Killer" is too cool to kill, so we'll just let them both live. Oh, Plus, we'll let the villain save face and get a heroes death too. " Come on.
The much better ending would have been Killer coming back and killing Driver after Driver kills Cop. Then Killer could pull the dollar out of Cop's pocket and walk away. Much more satisfying ending for the movie, especially since it fits with the theme. They probably didn't want to do that though because KIller was meant to be liked too.
Honestly, the best part of the movie was the Yin and Yang that Driver and Killer are too each other, and the movie would have been 100% better if they interacted more.