Expain how and what from the Catcher in the Rye instigated the killers such as Mark David Chapman, and John Hinckley Jr etc.
If you are asking how Catcher in the Rye influenced the activities of Hinkley and Chapman--two convicted murderers (attempted murder of President Ronald Reagan 1981; murder of John Lennon 1980)--the critics of Salinger's novel were quite clear about that at the time of publication.
They lamented that a novel would so graphically and with such detail portray the anguished life of such a tormented, anti-social teenager. Libraries refused to lend it to teens; schools refused to authorize it; parents were in a rage over its unproductive influence.
As a result of the criticism, Salinger retired from novel writing, left New York for a reclusive life in New Hampshire. In short, the novel provided a roadmap for impressionable youths and young adults to the path of rebellion, unrest and mental instability. This is how to define the influence the novel may have had upon Hinkley and Chapman.
T. Morris Longstreth stated in the Christian Science Monitor: "Fortunately there cannot be many of him yet. But one fears that a book like this given wide circulation may multiply his kind— as too easily happens when immorality and perversion are recounted by writers of talent whose work is countenanced in the name of art of good intention." (eNotes)