One of the biggest WTF moments in The Last Jedi came from how the filmmakers decided to shoot Luke Skywalker's death scene. After discovering that his appearance on Crait was merely a projection used to give the resistance enough time to escape, the movie cut back to the Jedi, staring into the sunset, as he disappeared from the mortal realm. It was obvious he was indeed dead, but many fans wondered what exactly it was that killed him. Was it simply the toll of using so much Force energy? Did the wounds suffered by his astral form effect his real body? Had the screenwriters just given up entirely?
Thankfully, Mark Hamill himself has gone into a bit more detail on the moment over on Instagram, explaining that it was indeed the Force which ended him. On the social media site, he wrote:
"THE FORCE KILLED LUKE. You have to acknowledge the irony in his fate. Almost like an addict that kicked his addict cold-turkey, remained clean for decades, only to re-use just once and then, tragically, overdoses."
While it was always speculated that exerting so much effort to project himself across the galaxy killed him, this quote does add a rather unique dynamic to the act. It implies that had Luke not cut himself off from the Force after failing Kylo Ren and the rest of his students, then he probably would have been powerful enough to perform the act without sacrificing himself in the process. It also raises a couple of questions about the nature of both the Jedi and the Sith as well, and how damaging it can be if they're not attuned to the energy that flows through them.
They're questions for another time, however, and for now we can be content with a more solid explanation of one of The Last Jedi's more ambiguous sequences.