The implication is that Christianity itself made them cruel, and it's true. Slaughtering heretics was logical if you thought that their influence would condemn people to hell. The Bible says you shall not suffer a witch to live. You can't say atheism made anybody cruel, since it has no doctrines. If Christians and atheists are equally cruel, then that proves Christianity is worthless for making people better morally. The whole presumption of the Abrahamic religions is that it makes a better society. Meanwhile, the believers in the USA openly advocate against helping the poor through any organized system that doesn't abide cheating. They talk a great game about charity, but that's voluntary, you can still be selfish. In any case, if charity worked we wouldn't need welfare.