Muhammad (if he existed) in my view was worse than Genghis Khan, at least in terms of intent. Both essentially did whatever they wanted, yet millions revere them. If someone like that is your external authority, and you don’t bother to think if an action is right or not, how can you claim it is inadequate if a person doesn’t follow an external authority?
The Abrahamic religions have the story of “God” asking Abraham to kill one of his sons, and Abraham being willing to do so (that he didn’t have to in the end is irrelevant to my point). The point of the story in these faiths is to show that blind obedience to “God” is what a good follower of “God” would do.
That illustrates to me that in these religions, there is no act so immoral that a religious person would not do, if they believed “God” told them to do it. It happens all the time that people believe “God” (or a “voice in their head” told them to). Nevermind prisons, there are a lot of mental asylums with such people, despite them supposedly following an external authority.
If "God" could tell you to kill a child, how can you tell that a good being is talking to you? Essentially, you think people should ignore their conscience, and follow somebody else's. People like Genghis Khan could certainly have a bad set of morals according to wider society, but that's the point. People's consciences are often different from society's. In Genghis Khan's case, he may have actually acted normal for the culture he lived in. I believe that eternal torture is immoral for example, which is contrary to what billions of people who have lived throughout history have believed.
The story of Muhammad is rather similar to Joseph Smith, both Islam and the Later Day Saints claim to have had prophets who received messages from God, via an angel, who then told other people what was allegedly said. Anyone could claim to have received such a message.
The Abrahamic religions believe that God killed almost all life on Earth in a flood. Apparently this is not a wrongful and criminal act.
There was an experiment where researchers dropped wallets with money in them in cities around the globe, and calculated what percentage of them were returned to them. The "Muslim countries" in the study didn't do especially well in terms of returning the wallets. Many more secular countries did better.
https://www.science.org/doi/10.1126/science.aau8712
In Christianity and Islam, there is belief in eternal torture in Hell as a punishment. I regard this as the most immoral act imaginable, yet billions follow these religions. Do they do so out of fear, or because they think this is acceptable?
The Nazis killed millions of Jews and other peoples in the Holocaust, then burnt their remains in furnaces. There are stories that some people were thrown into the furnaces while still living. Anne Frank died at one of their concentration camps of all illness, so she didn’t suffer that fate. I don’t know if you’ve ever read her diary, but if you have, you’ll know she was one of the most humane people who ever lived, who’d never dream of hurting anyone. Yet if Islam is true, Anne Frank will be tortured in the fires of Hell for eternity for rejecting Islam, as she was a faithful Jew. Islam and Christianity therefore to me are worse ideologies than Nazism.
People like HÍtler and Genghis Khan and Muhammad (if he existed), probably did do essentially what they wanted, but then so did people like Immanuel Kant and Baruch Spinoza (who basically hurt nobody in their lives). There were also people who felt duty-bound to kill thousands of people in the name of their religion they followed (which they didn’t invent), in crusades and jihads etc. – these people will be rewarded with eternity in heaven, if their religions are true.