Before the 2nd World War
During his rise to power, he led the fight to end the democratic system of government in Italy. He believed instead that Fascism was right for his country, and for all the world. He was of course, direly wrong. Before the second world war, he ordered the invasion of Ethiopia. This campaign has been especially noted due to its brutality, and the imperialistic nature in which the country was attacked. He even used mustard gas.
During the 2nd World War
Mussolini helped Hitler as one the leaders of the axis powers. When the allies invaded Italy, the decision was made that he must be removed from power. Many resistance and partisan groups led the fight against Il Duce They deposed Mussolini, arresting and imprisoning him.
Half the time he was screwing around with his super-expensive Amati violin, anyway.
His Nazi allies from Germany sprung him from goal. After leaving prison, he continued to lead the few sections of Italy that had not yet been liberated by the American and British forces. Finally realizing he had lost the war he made an attempt to flee the country, and to exile himself in Switzerland. His enemies caught him however, and executed him along with his mistress and hung their bodies upside down in the village centre, so as to signal an end to the terror.
He is widely considered to be the bitch in his relationship with Hitler.