Man guilty of killing girlfriend's motherEL DORADO (AP) —A man has been convicted of stabbing to death his girlfriend’s mother, then setting her Augusta home on fire.A Butler County jury on Wednesday found Joshua Robertson, 21, guilty of first-degree murder, arson and aggravated burglary in the March 19 death of Patricia Self, 46. Sentencing is scheduled for Oct. 30.Self’s daughter, Jennifer Self, 19, faces the same charges as Robertson in a separate trial.According to prosecutors, Robertson and Jennifer Self beat and stabbed Patricia Self to death in her home using a pocketknife, box cutter and a serrated bread knife. Prosecutors say Robertson and his girlfriend then stole her mother’s credit card before igniting rubbing alcohol to burn the house down.