First up: spoilers for Logan if you've not seen it yet. This isn't hugely ruinous but it will discuss the Westchester incident and how it relates to Professor X.
In Logan the Prof is suffering from an unnamed degenerative brain disorder causing seizures that paralyse everyone around him. Several times during the film there are references to a previous incident at Westchester, with the implication that, at least once, this condition caused something bad to happen. (And it's worth pointing out that Salem, Westchester, is where Xavier's School for the Gifted is/was located).
Speaking to comingsoon.net writer/director James Mangold has explained more about this passing reference, explaining that the film originally started with the Westchester Incident, and an elderly Xavier killing all the X-Men during a seizure.
"I wrote that scene. I wrote it, and at one point, it was even the first scene in the movie," says Mangold. It didn't last though because, as he explains, "it suddenly made the movie about X-Men dying, as opposed to allowing the movie to be a kind of unwinding onion, allowing you to enter the story and go, 'Where is this going?'"
Even mentioned in passing, as it appears in the finished film, it's clear the event was a major part of both Logan and Xavier's past. However, in the early drafts where it featured, "it loomed so large, and I felt like it also was falling into the formula of the movies, with the big opener, that is setting up the mythology first," says Mangold. Instead, he decided, “what if we do an opener that leans into character first? Actually underplay those things?” Let them just feel like it’s more like a normal thing, like it’s happened. And instead of underlining it, yeah. Just let it live in the background."
