An Oklahoma man convicted of killing his ex-girlfriend and her 7-month-old daughter after setting their home on fire nearly 20 years ago was set to be executed on Thursday. Raymond Johnson, 52, was scheduled to receive a lethal injection at the Oklahoma State Penitentiary. He was condemned for the June 2007 death of 24-year-old Brooke Whitaker and her 7-month-old daughter, Kya. Prosecutors said that after Johnson and Whitaker got into an early morning argument at her home in Tulsa , he repeatedly hit her over the head with a metal claw hammer. Whitaker’s skull was fractured and she had more than 20 lacerations on her face and scalp. But she was still conscious and begged Johnson to spare her and Kya, who was sleeping in a bedroom, prosecutors said in documents prepared for Johnson’s clemency hearing in April. “She begged him to call 911. She begged him to let her mom come get baby Kya. She begged him to think of her children,” the Oklahoma Attorney General’s Office said. Whitaker had three other children.…