Actually, he did not have sex with a nine-year-old. Aisha was approximately sixteen when they got married. She was the daughter of his best friend, who wanted their relationship as friends to become familial. At age nine, he decided that his daughter Aisha was to marry Muhammad, but they had to wait until she was fifteen to gain her consent. Back then, fifteen was the age at which you were considered a full member of society for both males and females, i.e. you were allowed to marry or participate in warfare. So, even in that society, it wouldn't be acceptable to marry a nine-year-old. As for massacring Jews, the story wasn't that simple. A certain tribe of Jews called Bani Quraytha had betrayed the entire society of Medina and violated the Constitution of Medina by allowing the enemy to enter Medina from their side of the city, which was fortified from all sides except their's, since they had agreed to guard it. When the battle ended and they were discovered to be the betrayers, they were offered to pick a judge to sentence them, so they pick a man of their own who had recently converted to Islam, Saad bin Mu'aath. He asked them which law they wanted to be judged by, the Islamic or the Jewish. They selected the Jewish, and according to the interpretation of the Torah at the time, the punishment of treason (which to this day is regarded as the most heinous crime by Jewish society, and is punished most severely in Israel) was the execution of the adult men and the enslavement of the women and children. Also, he did not simply slaughter all the Jews, only those in Bani Quraytha, and only those who chose to remain loyal to their tribe.