How can a healer be a villain?

"Healer? We don't need that in our party. Sorry" "Omg, that's useless job. Just go remake your character. Hopefully you will get another job" In the virtual reality game, Nana (IGN: Megumi) got a job as a healer. But this job is hated by all player, that no one willing to take it. Annoyed, Megumi determined to become the number one healer and will definetely slapped all the player who always mocked the healer job!