Once kylo fell luke made no attempt to turn him back or to help others like leia or han turn him back. It was a Pandora's box. It speaks volumes about the gravity of the situation when Jedi Master Luke Skywalker can't even turn him back. Wounds were too fresh and it was one that wouldn't heal until Luke mustered up the courage and realisation enacted by Rey's visit.

Yet luke had that whole moment of weakness because of what he saw in a vision. And it was that vision that caused luke to think he couldnt be redeemed. It this instance, said visions were true.

Which is why luke could have assisted leia or han in redeeming him. There was certainly more he could have done than just running off. Such as what? Do you think he'd run off to face Kylo just after witnessing the murder of countless students, a burning temple and two very, very disappointed parents that happen to be your sister and best friend to apologies to? Hell, I'd run away to.