Muhammad's conduct is considered ideal for Muslims:

You have indeed in the Apostle of Allah a beautiful pattern of conduct for anyone whose hope is in Allah and the Final Day, and who engages much in the Praise of Allah. (Qur'an 33:21)

But is he an example of a person with great conduct? To consider this, we should note that everyone sins, but if someone continues in serious sins and does not repent, it would be difficult to consider that person righteous.

Child Marriage

The first thing we should consider about Muhammad is that he married a six-year-old girl. The little girl was called Aisha.

The Prophet wrote the marriage contract with Aisha while she was six years old and consummated his marriage with her while she was nine years old and she remained with him for nine years (i.e. till his death). (Sahih al-Bukhari 5158—Narrated Urwa)

Should a man in his 50s be having sex with a little girl? No, and lest you fall for the lies that Aisha was really much older, see Sunan Abu Dawud 2116, Sahih Muslim 3311, and Sahih al-Bukhari 3896. In addition, you will hear that Muhammad was waiting until she reached puberty, but in reality, Aisha fell very ill when she was given to Muhammad.

Did Muhammad repent of this? Far from it, it seems. He is said to have had sex with all of his wives in one night, according to Sahih al-Bukhari 5068, and he never prohibited paedophilia among his followers. Yes, he permitted his followers to have sex with girls who had not reached puberty.

The Qur'an describes the how to measure the post-divorce waiting period (iddah) of females who haven't menstruated:

And those who no longer expect menstruation among your women - if you doubt, then their period is three months, and [also for] those who have not menstruated. And for those who are pregnant, their term is until they give birth. And whoever fears Allah - He will make for him of his matter ease. (Qur'an 65:4)

A tafsir confirms that this passage is talking about pre-pubescent girls (Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs).

For more, listen to Does Islam Allow Child Marriage? With Dr. David Wood.

Brutality

Muhammad exhibited a pattern of cruelty. Individual bad acts cannot prove that a person had bad character his whole life, but a consistent pattern of horrid behaviour can. On that note, he once tortured a man for money (Ibn Ishaq, Sirat Rasul Allah, p. 515). He also permitted the rape of slaves and women taken as booty after battle (Wiki Islam article). Furthermore, he robbed caravans to acquire wealth (Myth: Muhammad Raided Caravans to Retrieve Stolen Property), kept and sold slaves (Myth: Muhammad was an Abolitionist), and butchered conquered people, besides many who insulted him (List of killings).

Overall Character

While he could be charitable and decent in some ways, he married his own adopted son's wife (Surah 33:37; Surah 33:4-5). He had many wives. He amassed wealth. He owned slaves. Was this man selfless or selfish? He married a little girl (who was a best friend's daughter), slaughtered Jews, and endorsed rape. Was he a man who made some mistakes or one who consistently did evil? The answers are obvious, and I hope you conclude the same after some investigation of your own.