Joseph Cornelius, a 47-year-old homeless man, is accused of killing and sexually mutilating an 11-year-old Pittsburgh boy.  He said he killed him for trying to take his radio, and then he butchered his body to make the crime look like the work of a pedophile.  He told investigators he was drinking beer when Scott tried to take his radio.  He said he became enraged, choked him and when he realized he was dead, hid the body in some bushes, went to a bar, returned and mutilated the body.   Cornelius admitted killing Scott Drake and gave details of the crime only the killer would know.  The boy was found dead in a grassy area on the city's North Side on Monday.  Cornelius told police he cut off the boy's genitalia to imitate a mutilation case he had heard about in Oklahoma.  According to his confession, he used a broken bottle to mutilate the body and threw a knapsack containing the severed body parts into the Allegheny River.