No one really likes Nice Emma. Evil Emma is the sexy one. So she needs to be maneuvered into looking like a bad guy, but is really a good guy so that fanboys can vicariously do her through the medium of a geek like Scott. She'll explain it all later when she uses her power as Dark Queen of Mutantkind to break the X-men out of HAMMER camps.
Also, it raises the hopes of Emma/Scott haters like me. That's why I know it's a bait'n'switch. Because Marvel hates me.