Mutants are fighting now because if they don't stop the cloud in two weeks every single mutant on Earth will die a horrible and painful death and the Inhumans could care less about it, and Scott died after the X-men knew that the cloud was killing mutants, months after but yet the Inhumans only moved them around but did nothing to actually stop the cloud, which is not vital to their future, Inhumans exist without the cloud so they don't actually need it to survive...the royals seem intent on just waiting it out until all of the mutants are dead, Medusa didn't seem that concerned with their health, her hope was to have them on her side but if not she didn't seem broken up at the thought of them not helping she and the Inhumans either...