Growing up, one of our family rules was to calm down down before confronting one another. In other words, not to speak to each other when angry.
That was a good rule because problems were much easier to solve—and feelings less likely to be hurt—when we talked things over and practiced self-control.
However, as an author, I have found that writing when angry has had a dual positive effect.
First of all, on a personal level, it has been an excellent way to vent—to get rid of a lot of negative feelings. (Many of you most likely have heard of the cathartic effect of writing a letter to someone—getting all of the raw emotion out with pen and paper—even though you never intend to send it.)
Secondly, writing when angry can do wonders for your story if you are writing a very emotional scene. All of that energy just flows through your fingers onto the keyboard and fills the computer screen with unbelievable intensity!