When it comes down to it Phoenix was a perfect copy of Jean even if it wasn't explicitly her, and never would have went dark if it hadn't been for Mastermind and Emma in the first place. Plus, Jean got all of Dark Phoenix's memories at the end of Inferno. Even if it wasn't something that the normal Jean consciously did she remembers doing it, and as was mentioned, Dark Phoenix was literally a psychotic break brought on by Emma and Mastermind, if anyone should be guilty, it's them.