The color red is associated with violence...That's why the planet Mars was named after the Greek god of war. It could have to do with the fact that its the color of blood. The outdated phrase "I saw red" means I wanted vengeance. It's not made entirely clear, but from the context it's safe to assume that the author is trying to say that the violence was escalating...in both sentence, the meaning of descends is actually the opposite of the word's literal meaning. to say that madness is descending means that a mob mentality was taking over the crowd, almost as if an atmosphere of chaos were descending from the heavens and coming to rest on the crowd, thus inciting them to violence.

Red mist descended I imagine describes blood floating down from the air. Madness descends probably means madness spreads across people. Hope that helps!