Originaly the Phoenix was a hitchhiker into Jean's body -- a cosmic spirit (for lack of a better descriptor) that joined with Jean during a crisis. Human bodies, even mutanat ones were not meant to hold such power, and so eventually the cosmic spirit that had no human morals 'took over' Jeans body and mind almost completely and she became known as Dark Phoenix.

Since the entire saga of why Jean was exposed to such a thing and why she needed to bond with the Phoenix powere was never part of the movieverse, they needed another reason for Jean to develop the Phoenix personality.