I've been involved in a bitter dispute with my Local Government Council for about three years on and off and, in that time, in various letters of complaints/emails, I've probably written about 100,000 words. Looking back at some of my earlier complaining emails/documents, I can see I was overly verbose and overly ranting, but I can see a pattern of development, a style, a punchier voice that has developed over time, making for more effective writing that I can use in my fiction. So if you ever find something that has made you angry, try to use it as way of improving your writing. I always have something to say on the matters of my dispute. If it really makes you angry, you'll never suffer from writer's block.