History
When the X-Men returned from the future, Jean decided not to block the memories of that time, believing that they could instead use that future knowledge to their advantage. For a time, she blocked Professor X from learning of this via an unexpected use of her own unblocked telepathic power. This quickly started to fall apart, however, as she took the knowledge of Magneto's attack on Manhattan from Xavier's mind, arguing that she could be trusted to use this power to safeguard the future.
A short time later, the X-Men went to confront Magneto in Manhattan. While the majority of the team was horrified when Marvel Girl wiped his mind with her psychic attack, they let themselves be convinced that it was for the greater good, and accepted Jean's lead after Xavier withdrew his support because of their actions. The team had moved into a new base at the top floor of a New York skyscraper that Angel used his inheritance to obtain for them.
They soon realised that their actions would likely lead to major problems for the wave of newly manifesting mutants they knew was about to occur, so they chose to travel around and rescue everyone they were aware of before they could become targets, starting with Kitty Pryde, Dani Moonstar, and Sam Guthrie. When they arrived at the Xavier School, the team was surprised to see Xavier playing chess with a partially cured Magneto. The Professor pointed out that Jean's current morals bordered on those of Eric's, although she denied the accusation. This was the point, however, that Angel and Iceman decided that they could no longer follow Marvel Girl's lead, and chose to stay at the school.
This status quo did not last for long, however, as Jean only became more controlling, going so far as to wipe the minds of the Beast and reporter Trish Tilby after the former made contact in an attempt to sway her to their cause. This was the final straw for Cyclops, as he no longer felt he could trust her. He picked up Hank and brought him to Xavier to receive the same treatment that he had been giving Magneto.
Soon, the X-Men were watching the news at the X-Mansion and saw Jean had merged with the Phoenix Force and was killing anti-mutant leaders on live television, even burning some alive. Realizing that she had to be stopped, Scott led every battle ready member of the team, which now counted Magneto among its number, to fight and distract her while the Professor attempted to reason with her on a psychic level. Sadly, this ultimately failed, and the X-Men perished when Jean lost control and disintegrated everything around her.