The beauty of this movie is that we dont just have sympathy for this man, we identify with him. Most people have felt the largeness of the world weigh down on us when we are ridiculed and punished for an embarrassing illness or Fir not having a father or for simply not making it in life and having our and dreams stomped on over and over again. This is not a matter of sympathy but of identity.
The poor man in the movie first killed in self defense and then started killing evidently as a result if an extreme situation coupled with being denied his medication out of cuts from the city of Gotham. The fact that a sigficant portion of the city resonates with his desperation only further shows that he is not necessarily wrong. This is a case of Marie Antoinette saying "let them eat cake" to crowd of desperate hungry people who cannot endure anymore wrath and poverty...
Although I will never go into a killing spree, I greatly enjoyed the movie, it describes a desperate situation too many of us have experienced unnecessarily. I dont just feel for this man, but identify with him. I believe many viewers will.