Star Wars: The Force Awakens Trailer 2 Breakdown - 26 Things You Must See

Prior to the shot of Finn minus his helmet that appeared in the first teaser (and does again here), there's a glimpse of him in full stormtrooper garb, complete with a bloody hand-print.

So either he's killed someone important, or he's watched someone die in close proximity. Either would be enough of a spark for him to defect, you'd suspect.