I view all dreams as providing me important information from my unconscious. I view nightmares as a backfiring to the dreaming process showing me that I am either under a great deal of stress in my waking life or I have ignored an important issue and my dreams are now "screaming" for my attention.
A nightmare is usually so unpleasant because it is pushing us to confront what we would rather not address. When I have a nightmare, I try to get past the anxieties and fears within the dream and attempt to understand what the message is. The nightmare will usually contain important symbols that indicate where I am having an issue.
Overall, I see nightmares as prompting me to address a problem in a different way. When I make a change in my waking life to address the problem, the nightmare tends to resolve itself.
It can be hard to identify what you are ignoring or repressing! If I am struggling to identify what I am not addressing, I attempt to resolve the story of the nightmare. For example, if I am dreaming of running away from a monster, I will visualize this scene of the nightmare and visualize myself confronting the monster instead of running away. This will either change my nightmare if it recurs or it will help me identify what I am running from in my waking life.