Apparently no one told Medusa that the Terrigen cloud was reaching total saturation and was going to kill every mutant on Earth. So, when they told her and then her own people gave her a way to destroy the terrigen, she did it without hesitation. She then basically said she would have done this at the very beginning, without fighting, if the X-Men had just talked to her. To her, while terigenesis is important to Inhumans its not worth the life of even a single mutant, much less all of them.
In the end, it was the X-men's stupidity, lead by Emma Frost being super manipulative (she even takes over Magento's mind and forces Forge to build super inhuman killing sentinels), and Emma being angry that the terrigen killed Cyclops before they knew that would happen that caused the fighting. So, this whole story would have been over about 4 pages into issue #1 if anyone had bothered to talk to Medusa about the terrigen cloud

This event is literally the biggest waste of time of any event I've read, and at this point I've read a lot of them. It also made all the X-men (except Beast) come off as idiots.