To get a better picture of who Muhammad was we should look to what Muhammad's contemporaries said of him. The fact that no biography of Muhammad exists that was composed by a contemporary doesn't mean we don't know what they thought of him. In fact we have the words of Muhammad's contemporaries in the koran. This makes it all the more damning for Muhammad. Even people of his day saw through his lies and saw what laid behind his message.
"He is liar and a magician."
"He is possessed by devils."
"He is an insane plagiarizing poet."
"a lunatic, a farfetched forger, a fool, a specious pretender, and deceitful."
"The unbelievers said: 'He is a deceiving sorcerer, turning many gods into one deity.'"
"motive behind the Qur'an. It is a fabrication."
"They laugh at the Qur'an."
"Allah hasn't sent down anything. You are only speaking lies."
"This is nothing but pure sorcery."
"The unbelievers say, 'The Qur'an is nothing but earlier people's lore.'"
"These are fables of antiquity which he has reinvented."
"What sort of prophet is this? Why was no angel sent to him?"
"These are only confused dreams. He has invented them. Let him bring a miracle to us as the earlier prophets did."
"They say of the prophet, 'He has forged the Qur'an.'"
"We find him full of folly and a liar to boot."
If you turn to the Bible to see what the contemporaries of Jesus thought of him you get a message of compliments and ,from the Pharisees, insults. The difference is that the Pharisees had a personal stake in the success of Jesus' message. Their bias is easy to see in the Gospels.
The people of Mecca would not have cared what Muhammad taught if it had not been insulting, degrading, disgusting, and violent. In fact their are several times in the hadith that mention them simply wanting Muhammad to stop harassing them. They would have allowed him to teach. They just wanted him to do so peacefully.
The greatest difference in their dealings with their contemporaries is how they responded to the insults thrown their way.
Mat 12:32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the [world] to come.
"Some of them talk ill of the Prophet. For those who offend Allah’s Apostle there is a painful punishment. Anyone who opposes Allah and His Prophet will abide in hell forever."
By the hadiths' own admission we find a disturbing and disproportionate response to the Meccans that disagreed with Muhammad.
Ishaq:130/Tabari VI:101 "'The nastiest thing I saw the Quraysh do to the Messenger occurred when their nobles assembled in the Hijr"
"We have never seen the kind of trouble we have endured from this fellow. He has derided our traditional values, declared our way of life foolish, abused and insulted our forefathers, reviled our religion, caused division among us, divided the community, and cursed our gods."'"
So in other words. The nastiest thing they did was get upset at his violent attitude and behaviour. How dare they? If you need further convincing of the disproportionate response of Muhammad to a few insults:
"'We have endured a great deal from him.' While they were saying this, the Apostle walked up and kissed the Black Stone. Then he performed the circumambulation of the Ka'aba. As he did they said some injurious things about him. I could see from the Messenger's face that he had heard them. When he passed a second time they made similar remarks. When he passed them the third time, the Prophet stopped and said, 'Hear me, O Quraysh. By Him who holds Muhammad’s life in his hand, I will bring you slaughter.'"
Do we really still need to wonder why muslims kill people who have the audacity to reject Islam as truth or go so far as to call Muhammad a "murdering, thieving, pillaging, raping, ignorant, disgusting, pedophiliac." The list could go on but I think the point is made.
God bless
