(Disclaimer- All the sources are mentioned in the answer, and the answer has no biases attached.)

Since his non-Muslim Arab contemporaries criticised him for preaching monotheism in the 7th century AD and the Jewish tribes of Arabia for what they saw as unjustified appropriation of Biblical stories and characters and vituperation of the Jewish faith, Muhammad has come under criticism. Due to these factors, he was frequently referred to by the disparaging moniker ha-Meshuggah (Hebrew:, 'the Madman' or 'the Possessed') in medieval Jewish writing.

Given that Muhammad was frequently viewed in Christendom as a heretic, various Western and Byzantine Christian philosophers during the Middle Ages believed him to be the Antichrist, a perverted, despicable man, a false prophet, and even the Antichrist or being controlled by demons. Some of them, including Thomas Aquinas, condemned Muhammad for the way he handled doctrinal issues and for promising eternal hedonistic pleasure.

Muhammad's genuineness as a prophet, his morality, his marriages, his ownership of slaves, and his psychiatric state have all been the subject of modern scrutiny. Muhammad has also been charged of sadism and violence toward his adversaries, particularly when the Banu Qurayza tribe was invaded in Medina.

Byzantine records, which were published not long after Muhammad's death in 632, provide the oldest known Christian knowledge of Muhammad. One of the Jews participating in the conversation found in the Doctrina Jacobi nuper baptizati says that his brother 'wrote to [him] alleging that a false prophet had appeared amidst the Saracens.' 

Regarding Muhammad, another Doctrina adherent responds: “He is dishonest. You will learn nothing true from the aforementioned prophet other than the need for human sacrifice, for do prophets come with swords and chariots? ”.

Theophanes the Confessor, a 9th-century author, is another Greek source that mentions Muhammad. John bar Penkaye, a writer from the seventh century, is the earliest Syriac source.

Muslims frequently neglect the less commendable traits of their prophet in favor of concentrating only on his positive traits. Muhammad started plundering caravans after leaving Mecca, as we've already witnessed. Thus, one of the main driving forces behind people's quick conversion to Islam was soon greed. In fact, Muhammad purposefully enticed people to Islam by using the spoils of war. 

'Are you distressed in mind because of the wonderful things of this world by which I win over a people that they may become Muslims while I entrust you to your Islam,' he responded when chastised for the way he distributed his newly discovered wealth.