X-Men: What Happened To Everyone Who Became The Phoenix?
3. Scott Summers
Scott Summers' time with the Phoenix Force is one of the most devastating versions of the character to ever come out of Marvel.
When he's joined to the Phoenix he goes mad with power and then does the unthinkable. He kills Professor X.
It doesn't start out this way, as the five attempts to make a perfect world utopia, but soon enough the power goes to their heads and people live in fear. When he's attacked by the combined forces of the Avengers and the X-Men he's left behind, he transforms into the Dark Phoenix after straight up murdering Charles Xavier.
It doesn't end well for old Slim however, as he's weakened enough for Hope Summers to absorb the force for herself before she warps reality back to how it should be.
Scott's arrested and thrown into prison but he isn't in there long as Magneto, Magik, and Danger break him out. He heads up a splinter version of the X-Men whose agenda is far more extreme than the dream Charles once had for them. Tragic.