It is a rejection of the entire moral system of Christianity

It's worse than that. That is where it starts because they can use examples of religious tyranny to push the claim that morality is tyranny. But they claim that there is no absolute right or wrong. That all morality is personal opinions being foisted on others.

They do this because the people who believe that won't challenge evil. Even when they know something is wrong they hide behind the excuse of "it's not my right to interfere". It takes a lot of courage and strength to stand up to evil and an excuse like that will keep a lot of people from summoning that courage. People need to feel shame when allowing evil.