Johnson told Hillsborough County deputies that he was on his way out of the door after he and his girlfriend got into an heated argument over changing a channel to watch a football game.
He said that he started packing his bags in the master bedroom when his girlfriend allegedly walked in and began “verbally berating” him.
The boy, who is not Johnson’s son, then accused the 42-year-old of hitting his mother.
Johnson allegedly pushed the child, grabbed his handgun and began firing at his girlfriend while she was holding a PlayStation. He shot and killed the child, who had walked out, police said.
Johnson faces several charges including murder and aggravated child abuse in the death of his girlfriend and her 10-year-old son, neither of who were not identified.