Quranic believers try to paint Muhammad as a Saint - often they try to make out that he was sinless. Well this is patently untrue.
What is so horific about this man was the way he tortured and killed people in the most gruesome ways and thought nothing of it.
Here is one example of the deeds of this man who Qur'anic believers put on par with their "god" in their statement of faith the Shahadah.
During the year 628 A.D., eight men from the clan of Uraynah, came to Medina. They became Muslims. While there they got sick. Muhammad prescribed a medicine for them: he told them to drink camel milk and camel urine. After the men got better, they brutally murdered the shepherd of the camels, stole the camels, and tried to escape. Muhammad sent an armed party of 20 men after them. They were caught and brought back to Medina. There, Muhammad pronounced their judgment: their eyes were branded with hot irons, then plucked out, their hands and feet were cut off, and then, while still alive, they were thrown out on the hot desert to die.
One can read the whole account here:
Justice and
Mercy