To be fair, there are people who have odd religious beliefs, but no one imagines that they will actually kill children.
MOO, the children were killed, because LVD wanted their money, and they didn't fit in with her plan to live in Hawaii. She killed JJ, just out of pure hate towards Kaye, and Charles Vallow.
It had nothing to do with religion. That was just some sort of justification made up to appease their narcissism. Keeping up the illusion CD and LVD were so holy and wonderful. Even to themselves.