Chapman has yet to provide a clear motive. Based on the Bresler book, Who Killed John Lennon?, I am starting to think that this was because he was being mind-controlled by someone else and acted out of some sort of order from a controller.
To support this assertion, Bresler makes a stunning discovery; namely, that Chapman did not make his final trip from Honolulu to New York, as has been commonly accepted, but to Chicago, where he spent three days prior to going to New York. Something happened in Chicago.
Then, after the assassination, when the authorities went to Chapman's hotel room, where he had laid out all kinds of evidence of what he had done, the flight information had been altered, so that it looked like he flew directly from Honolulu to New York. Bresler doesn't think Chapman did this.
If so, it opens up a huge Pandora's box.