The same Luke who nearly killed his father in a fit of rage, but stopped at the last second realizing what he was doing was wrong. What made Luke great wasn't that he was some kind of light side savant, it was what Yoda said "Once you start down the dark path, forever will it dominate your destiny, consume you it will." that's where the Chosen One was great. The Chosen One could look at Dark Side and turn away. It didn't consume them. Luke flirted with the Dark Side, and then realized what he was doing, then stopped. Just like he did with Ben.
What would you want Luke to do? Go to the throne room of the First Order fighting through their fleet to talk down someone who thinks he wants him dead. Someone who's already been turned by Snoke. What better way would you have handled that?
Also Luke running away and abandoning his nephew as you say wasn't done in Ep 8, it was done in Ep 7. We'll come back to the end of Ending of Episode 8. Why did Luke do what he did? He robbed Ben of a chance to do things that would have pushed him to the Dark Side forever. He wasn't put in a situation where he would have to kill his Mom and the resistance. Stopping if you would his Slaughtering the Separatist Council Moment that Vader had.