Technically, she never really had any beef with Batman or was intending to kill him, she was only after the Joker and the mobsters, so once that job was done, she leaves.
Something I really loved about the movie, is how bittersweet the ending of it all is, that for all intents and purposes, Andrea wins, she (as far as the movie is concerned) kills Joker and everyone else in that mob is dead, but then realizes how lost she is with no real other goal or drive to thrive for. Meanwhile while Batman loses, unable to save Joker or Andrea, he still has his mission he continues to fight for, and the film doesn't really say if thats good or bad, it just is.