Who's the old man he killed at the beginning? He looked like a Jedi.
Took me my second viewing to get what Ren meant by "you're so right."
He's not denying where he came from. In his mind he's celebrating it. He's the grandson of the most powerful Sith Lord.
His name was Lor Sen Tekka. In his youth, around the time of the Clone Wars, he was a galactic adventurer and explorer, and a believer in the ways of the Force as espoused by the Jedi, though the Force was not with him in such a way that he could hope to be a Jedi Knight himself. After the fall of the Jedi, he worked to preserve what of their knowledge he could, which is how he came to know Luke and Leia. The village on Jakku that he was living in was a commune of Force worshipers such as himself.
He's signed on for at least one more film
For backstory purposes most likely. Inevitably he will have been the one who rescued Rey from Luke's academy when Kylo went on his rampage