It was surprising to see Luke Skywalker throw his father’s lightsaber over the cliff in Episode 8. Rian Johnson explains that it was parallel to Episode 6 when he tosses his own lightsaber aside, choosing the light over darkness. In this instance, he’s choosing to go down a different path than down the wrong path with Anakin’s lightsaber.
Who knows what Luke’s been dealing with for the past years, I’m sure he’s had a run in with his demons just as much as any Jedi has. It’s difficult to completely stand in the light, he could’ve even felt his own pull to the dark side. After all, Luke did use dark side force powers like force choking the Gamorrean guard at Jabba the Hutt’s palace. All in all, it was definitely a shocking scene and was a very daring move on Rian’s part.
Who know’s maybe we’ll finally get to see what really did happen to his green Lightsaber in Episode IX. Even though we’ll more than likely see Luke as a Force Ghost, it will be nice to maybe have some closure finally.