[Scary]

Luke was a Jedi, now he is a grey Jedi, as he became disillusioned with the Jedi teachings, after The return of the Jedi he went to old Jedi temples throughout the galaxy looking or holocrons and what he learnt he knew not to be true, that is why as explained in the force awakened he went in search of the first Jedi temple to gain the knowledge of the original Jedi.

the original Jedi were known as Je'daii Order where there was no light or dark side only balance.

this is why at the end of the trailer he says the Jedi must end as he now knows that the only way there will be peace is if the jedi and sith orders either die or go back to the original teachings.