So the fact that Ethan's behavior was so different in this episode's new timeline had me wondering: did the Ethan we know from earlier episodes get "infected" as well? When Dogen said it was contagious, does that mean from person-to-person? If so, could he have picked it up from Ben (since we know they hung out all the time as kids)? Or he could have been revived at some point as well, obviously.

I would just chalk up the difference in behavior to the fact that he did most of his growing up off the Island.

The Ethan from season 1 certainly seemed to fit Dogen's description of darkness overcoming him. Especially if you contrast his scary behavior in that season to how he has been portrayed in every appearance since then, including in tonight's episode. It will be interesting to see if something like that is confirmed.