Distraught at having lost his final student - and compounded with the pain of losing Leia and Han - Luke snaps and force-murders an entire First Order battalion, along with some innocents thrown in for collateral damage. With everyone close to Luke now dead, the movie ends with audiences having no idea how he can be redeemed in Episode IX.
ok I'm done jerking off
he embraces the dark side and becomes a vicious killing machine with no one able to stop him