I was just reading a Snopes article about this billboard in Florida reported by Tucker Carlson's news blog, the billboard reads "The perfect man" and then lists some supposed things he did like behead 600 jews in a day, marry a 6 year old, own slaves, and stuff like that; but Snopes is claiming that many of these things are more contested than I, a formally very edgy in your face atheist, might have once believed. The article claims that basically none of this stuff is written in the Koran and the rest is gather from dubious sources that wrote about him 200 years after he died.
So academically speaking, what all do we *really* know about Muhammad and what he did? If someone tells me that Muhammad tortured and killed non-believers like this billboard says how skeptical should I be of that claim?
There are several sources you can go to. The Qur’an isn’t really a good source for Muhammad’s life, it’s not a biography like the Gospels are and usually only contains allusions to events that were occurring. The main chunk of information we have comes from the hadith. These are sayings of Muhammad which come from people who knew him at the time and are collected in the Islamic holy texts of Sahih al-Bukari and Sahih Muslim. Some of these are more reliable than others and scholars sometimes spend their lives verifying hadith. The other main source which may be more interesting to the non-Muslim are early biographies. For example Ibn Ishaq’s biography of Muhammad was published in the early-to-mid 8th century and contains the stories you mentioned about the killing of 600 Jews. You might think this is quite a late biography but for context the earliest biography of Alexander the Great that copies of which survive come 400 years after his death.