He was training a new generation of Jedi. There was no one else left to do it, so he took the burden on himself. Everything was going great, until...one boy, an apprentice, turned against him and destroyed it all. Luke felt responsible. He just...walked away from everything.

Ben "destroyed it all" not by killing everyone, but by burning down the gym because it was infested with vampires rats. Or the apprentice who turned was Snoke, not Ben. Or Luke's other apprentices escaped death and are in hiding with him, training at the first Jedi temple.

Basically, an exploration of any scenario other than the assumed "Skywalker turning to the Dark side = bad time to be a Jedi youngling."