Star Wars: The Last Jedi - 10 Predictions For The Biggest Ever Twist
6. Luke Killed His Students

The Theory
So far, we're being led to believe that the mysterious Jedi School incident was a tragedy that turned Luke against teaching and drove him into self-imposed isolation. As Han Solo said "He was training a new generation of Jedi. One boy, an apprentice turned against him, destroyed it all. Luke felt responsible… He walked away from everything."
But what if that is only a half-truth?
Kylo Ren could have turned against Luke's teachings and fled without striking any blow against the School and could still have fundamentally changed Luke's thoughts on Jediism. What if, frustrated by the inevitability of the irresistibility of the Dark Side's pull, Luke lost his mind and killed his own students to start the end of the Jedi at the source?
He wouldn't necessarily be a straight villain - more an impulsive, emotionally-led idealist (which is exactly what he was, despite Yoda's warnings, throughout the Original Trilogy) believing he's doing the right thing.
Would It Be Shocking Enough?
Given that it would change every perception fans have of the aged Luke Skywalker, it definitely would be. It would also fundamentally play with the traditionally binary approach to heroes and villains in Star Wars movies, and challenge fans to think about extremism and intentions.
Sure, it would piss a lot of people off, too, but that's sort of the point.