The assassination of Lennon could have been avoided if Yoko emerged first from the vehicle. In OTL John Lennon first exited the vehicle and walked past Chapman toward his apartment, when Chapman opened fire shooting Lennon in the back several times. So perhaps if Yoko got out of the car first, Chapman gets a strange thought, Yoko Ono is a demon who has John trapped in her sirens call, instead of waiting for Lennon he turns and opens fire at Yoko, she dies the same way as John, massive blood loss.
Lennon becomes seriously depressed, and rarely is seen in public, holing himself up with his son Sean at the Dakota building. He will have to find a new mother, so to speak and May Pang fits that bill. He continues to write but his music becomes darker, and very depressing, perhaps creating a new genre in the process.