I never said such a justification existed. I said people have murdered and killed in the name of Christianity. And as a matter of fact, they have done so for more centuries than not. If you're being murdered, it doesn't help if your killer has his theology wrong. Religions are embodied in people, institutions like churches, and governments. If the people running things are evil, the words mean nothing.
You still need to explain how people can be evil if the "idea" of evil doesn't exist.
You ignore the difference between the people who kill in the name of Christianity (which is against the teachings of Christ), and those who kill for Islam (perfectly in line with the teachings of Mohammed and the Koran).