Why?
There are millions of people the world over who will take a religious text, or something they have dreamed up and written down, or just voices in their head, and use it as justification for abhorrent behaviour.
I have said before, but it bears repeating, it enables such people to look in the mirror and tell their reflection, "I am doing the right thing because I am a good person following the right path …".
That feels so much better than the reality, which means looking in the mirror and telling their reflection, "I am a psychopath with no moral code whatsoever, I am utterly irredeemably horrible human being and the world would be a better place without me and people like me in it."
It's just human nature, we all like to think the best of ourselves - some go about it differently than others.