She's actually not even evil in this movie but crazy and angry. The writers are making it the her powers make her lose a sense of self (rd the how they describe her). So she won't be the unredeemable villian but the X-man who lost her way and comes back. In the end she'll know who she is completely and be in control and then be an X-man again.

Its not like she's going eating star in this movie and cost 5 billion lives.
Fire Made Flesh