One, he gave up. He found the venture to be pointless, that ultimately Jedi have breed darkside users at times and that at other times were blind to their appearance and that the Galaxy is better off without them. Then enter young Rey. It seems like the "reluctant warrior teacher who gave up" trope.
Two, he found some form of an old old way, an order before that of the Jedi that he feels is a much better path and wants to teach Rey about it. No more Jedi Council or organization. Just a galaxy of force users and a system they can follow.
I'm more of a fan of my first paragraph than the second.
And that would be what I'd lean towards. But how do you account for Bendu's comments in Rebels?
So kill one just youngling and then go save the Galaxy?I am not so sure of that, there is always a bit of Ying in the Yang, but the Jedi and the Sith strive to eliminate even that form of balance.
No, but you need to be a Jedi to...... be a Jedi. Jedi are just one path, a relatively extreme path. and like any extreme path, it all but asks for an opposing force. Luke tried to recreate the Jedi, he created Kylo Ren, that is why there should be no more Jedi, they invite the dark by their very existence.
That's my Number One possibility. But I see as Rey as showing him that it is inevitable that there will be strong force users. The darkside will rear its ugly head and if there is no one to counter it, to bring balance, then you get Supreme Leader Snoke and his merry band of the First Order. We don't really know where the Sith come from in this new cannon. All we know is that for whatever reason, they wanted revenge.