Answer by Jim Ashby:
Let's start with common sense.
Do you think he began the Muslim Empire, conquering all of Arabia, with self-defense? I know Muhammad is a revered prophet among Muslims but, at some point, even Muslims have to admit that he didn't conquer Arabia — converting all pagans to Islam — with peace and tolerance.
I should note here that, unlike the idol-worshiping pagans, 'People of the Book' — mostly Jews and Christians — were not forced to convert to Islam. Instead, Muhammad subjugated them with dhimma contracts that forced them to pay the 'jizya' tribute tax and limited their religious activity and interaction with Muslims (they were not allowed to hold any position superior to a Muslim, nor could they work in government). Many Muslims point to dhimma as 'tolerance' but, of course, it's actually subjugation.
If you're not swayed by common sense, how about ahadith? There are many ahadith by many authors, only a few of which are considered 'reliable'. All the reliable ahadith clearly describe some noble qualities but they also detail the violent, intolerant, militaristic ways of Muhammad. Rather than provide a litany of such passages, I'll refer you to thiswiki, derived mostly from ahadith (including the most reliable ones).
There's a whole body of Islamic apologia, developed over the centuries, that attempt to rationalize Muhammad's violent history. But it doesn't matter why he raided caravans for camels, booty, and hostages for ransom or for sale as (sex) slaves. It doesn't matter why he attacked villages and towns and forced them to convert to Islam. Some entire towns surrendered immediately and converted wholesale rather than face the alternatives: fight or die. It doesn't matter why he had at least three poets assassinated for mocking him . . . very much like what happened with Charlie Hebdo. It doesn't matter why he did these things. What matters is why you should believe that the all-wise and merciful Allah would allow himself to be represented by such a man.
Many Muslims consider Muhammad to be the best of all role models. He's often viewed as the epitome of masculinity. His example was emulated by his followers as they spread the Muslim Empire across three continents (with blood-thirsty brutality in Asia, particularly India). And his example is still emulated today, by jihadis, terrorists and other extremists.