The Mutant Massacre was an important storyline in the X-Mens history. The Morlocks were a motley group of mutants who couldnt blend in with humanitythey had very obvious physical or deforming mutations. To protect themselves, they lived in tunnels beneath New York City. But during the Mutant Massacre, the villain Mr. Sinister sent his team of assassins, the Marauders, into the tunnels to exterminate the Morlocks. The event caused some long-reaching consequences for the X-Men, and big events of such a nature would be perfect fodder for film adaptation. It could also lead to the return of Sabretooth to the X-Men films, as hes a member of the Marauders as well. Although if Sinister engineered the Mutant Massacre, why could it pop up in X-Men: Apocalypse? For one, story-lines are adjusted when adapted. In the comics, it was the Hellfire Club who helped corrupt the Phoenix, but in the movies it was the Brotherhood. The comics featured a mutant cure created by an alien named Ord, but in the movies it was Worthington Enterprises. Plus, the idea of culling the weak is hardly far from Apocalypses wheelhouse, and he would definitely consider mutants who hide from humans to be in that category.