Gloria could have stopped this
No-one ever brings this up, and I don't know why. Chapman goes to New York to kill Lennon, can't track him down, rings Gloria and tells her "I was going to kill John Lennon, but your love saved me." He returns to Hawaii, and less than three weeks later he goes back to New York.
At no point does Gloria think "Hmmm, I wonder if he's going back to kill John Lennon, like he said he did less than three weeks ago? Should I tell anyone about that, warn anyone? Is his blatantly crazy behaviour anything to be concerned about at all? NAHHHHHHHHHHHH. I'll just make a cuppa coffee and pretend to be shocked when Lennon gets shot by my very obviously insane husband."
If this part of the story is true, and there is no reason to believe it isn't, Gloria Chapman bears some of the responsibility for the killing.