I like to write journal entries in the voices of my characters. I sometimes even do it for people who have hurt me deeply in real life. The challenge is that you have to discover something new about the person or character. If your exercise reveals only what you came to the page with in the first place, then you have not tapped into the empathy you are going to need to write the story you want to write. You really to have to want to understand that person, which means you may have to let go of that anger.