While Jean did commit suicide, it was to prevent a greater evil. The Dark Phoenix could not be contained and it had already been seen to be willing and very able to commit planetary genocide. In that sense, Jean was “taking it with her” so that it could not bring harm to any other inhabited worlds.
Also, it’s important to note, this was not Claremont and Byrne’s preferred ending. Originally, Jean was to have had her psychic powers removed and returned to live with the X-Men as a normal human (the idea being that it harnessed her powers, so with those gone, the Phoenix would be defanged and powerless). Jim Shooter demanded the sacrifice because he felt that the original scripted ending was not enough of a punishment for what Phoenix had done, and that it would be seen as a cop-out both morally and dramatically ( he described it on his old blog roughly like this: “it’d be like taking the German army away from Hitler but letting him continue to run the country.”).
