Even outside of the fact that Jyn Erso comes from a broken past and has rebellious tendencies, the final shot of the teaser really locks in the suspicion of her change of heart. She appears to be wearing dark Imperial garb. What's more, it bears a slight resemblance to Captain Phasma's cloak in The Force Awakens. Of course, in the same way Luke Skywalker disguises himself as a stormtrooper, it could be part of an elaborate plan. Or, you know, it could be because Jyn Erso switches sides.
There's something else to note about those words that echo through the end of the teaser. This is the shot we see just after the character asks, "What will you become?" It can't be coincidence . . . right? No matter what, we just can't help but notice that evil, mischievous gleam in her eye.
