Because the Koran says so.
…Elsewhere in Khartoum, a Christian mother of a 2-month-old baby was recovering of her injuries Wednesday, May 25, and remained destitute after she and her husband were attacked for leaving Islam for Christianity.
Omar Hassan and Amouna Ahamdi, both 27, said knife-wielding, masked assailants on May 4 attacked them after relatives learned that they had converted from Islam to Christianity.
Hassan told Compass Direct News agency that he and his wife were renting a house from her uncle in Khartoum, but he ordered them to leave after learning they had left Islam.
They earlier fled Nyala, 120 kilometers (75 miles) southwest of El-Fashir, for Khartoum in June 2010. In the latest attack, a knife pierced the palm of Ahamdi, who said her brother had already stabbed her three times in the stomach nine months ago, seriously injuring her spleen, after she told him she had become a Christian.