What might the author be trying to say by creating both Mildred's attempt to commit suicide and Beatty's actual death?
The quickest answer to your question would be that in this particular society, most would prefer death to life. It must have been that terrible to live in. But a closer look at each person will reveal even further truth.
Mildred had no concept of the length to which her life had really been destroyed. This society had beaten all thought, emotion, and desire for human contact out of her. If you think about it, those attributes are what we really live for. Her attempt was somewhat unknown to her. She had no idea how unfeeling she had become and would not have noticed if she died. Her attempt occured out of her own futility.
Beatty knew better. He is a man who had been well-read. He was living a double life and it killed him. He put himself in front of an angry Montag and antogonized Montag when Montag had a weapon of fire in his hands. Beatty knew the potential of a life with thought and emotion, but he could not live it because he had to play the role given to him as fire captain in order to stay alive. By the time he dies, I believe he dies with the peace that death would be better than the world they live in.