If you go to the wiki page for Logan, it states in the plot section that
They care for a senile Charles Xavier, who had inadvertently killed several of his X-Men in a seizure-induced psychic attack a year earlier.
But there's no citation for this, and from what I remember the movie left it intentionally ambiguous. However, it seems to be an accepted fact that the above is what happened. The movie itself is clearly based on the "Old Man Logan" series of comics, in which
Logan kills all of the X-Men, believing them to be villains attacking the school, when in reality it is a hallucination caused by Mysterio.
Obviously there are things, such as characters, that Fox can't include in their X-Men Movieverse, so some details get changed, but to me it seems to be a significant change to have the guilty party be Xavier in place of Logan.
Is there anything that might provide some clarification one way or another as to whether Xavier was
the killer of the X-Men, or merely the cause of Logan's hallucination?
The latter of course being more in keeping with the comic story?