Wikipedia mentions that Chapman became a Christian an 1971. What is not mentioned here is the antipathy between the Christians and the Beatles.
"Chapman recalls having listened to Lennon's John Lennon/Plastic Ono Band album in the weeks before the murder and stated: "I would listen to this music and I would get angry at him, for saying that he didn't believe in God... and that he didn't believe in the Beatles. This was another thing that angered me, even though this record had been done at least 10 years previously. I just wanted to scream out loud, 'Who does he think he is, saying these things about God and heaven and the Beatles?' Saying that he doesn't believe in Jesus and things like that"
"In 1966, John Lennon told reporter and friend Maureen Cleave, “[The Beatles are] more popular than Jesus now.” The American religious public was indignant with rage, burning Beatles albums, boycotting their performances, and even threatening to kill Lennon. Jesus, I’m sure, was thrilled by our zeal."
I'm sure the Christians don't like to think that this murder was committed by one of their own. Well, the Beatles' fans don't either. And the murder was more because of the Christians attitudes than because of Rock and Roll music.