What was one negative thing about Mohammed?

There are a number of negative attributes concerning Mohammed. Probably the most commonly cited was his temper. He had very little tolerance for those who disagreed with his view of politics. He was ready to massacre the Jewish tribe of Banu Qaynuqa until one of his apostles told him otherwise (allowing them to merely be expelled) and actually did massacre the Jewish tribe of Banu Qurayza. He did this for what he judged to be military treason; the Arab Jews he murdered disagreed.

There are numerous other negatives concerning Mohammed at the Related Link below, with citations to relevant Qur'anic Verses and Hadith collections.