When Lily says that people would rather die than forgive, she may be referring to several people in the novel.  One is herself.  She has been walking around with the memory of her mother's death and her part in it for ten years.  She has not forgiven herself in that time.  Though it is very difficult for her to come to terms with the fact that her mother left her once, it seems as though she forgives her mother more quickly and more easily than she forgives herself.  Once she sees the photograph of her mother lovingly looking at her as a baby, Lily knows that she will always love her mother.  As for whether or not Lily is right about what she says, it depends on the individual.  Look at T. Ray for instance.  He has never forgiven Deborah for leaving or Lily for what she did.  He might be someone who would rather die than forgive.