Erik was alive. And he had declared war on the humans. Using his abilities, he turned basically everything off. So much metal was used in today's tech and all of it was turned off. This included the sentinels. Which was the only real good thing about what he did.
Charles Xavier was also alive. His ship crash landed into the mansion. But he was okay. He summoned his X-Men to come to him. Which they did.
Charles and Scott had an argument. Rightfully so. Why did Charles choose one of, if not, their biggest enemy to run the school? Why not let Scott or even Jane run it? They were actually trustworthy.
A plan was made up. To defeat Bastion and stop Magneto. Groups were made up. One team made to find Magneto. Who suddenly came to the mansion. Who was flying outside.
Everyone went outside. Charles immediately tried to reason with his old friend. But Magneto wasn't having it. He was angry and hurt by what happened in Genosha.
"Who will join my side?" Magneto asked. Giving the X-Men a choice to follow him and not feel his wrath.
Rouge, who cared for Erik and was grieving Gambit, joined Magneto. Roberto decided to join Magneto as well. His own mother gave him up to the sentinels.
No one else was going to join him. Now it was your choice. Magneto looked at you, waiting for your answer.