Luke found out that Kylo was dabbling in the Dark Side. The conversation Escalated Quickly. Luke Falls. The Knights of Ren are slaughtered, except for Ben, who Luke just CANNOT kill, thanks to Leia's twin-bond intervention. Luke sort of 'wakes up', realizes what he's done, RUNS LIKE HELL so he can't hurt anyone else...because his Light Side students tried to STOP him and...well...

But Leia still believes he can be saved.

BRING ON THE TEARS.