TAMPA — Dressed in jail-issued orange, Justin Garwacki took the witness stand on Wednesday, a more muscular version of the man he had been in 2010, when Hillsborough prosecutors charged him with the murder of his toddler son.Asked why he had agreed to help the state prosecute his former girlfriend, Kara O'Connell, 26, for the same crime, he replied: "I felt it was the right thing to do for my son."It was also to his advantage. Facing charges that could send him to prison for life, Garwacki had made a deal with the state, agreeing to turn on O'Connell, with whom he has a daughter, in exchange for a promise that he would spend no more than 40 years prison. If he took the stand, prosecutors said they would recommend 30.The first day of testimony in the trial of O'Connell, who has pleaded not guilty to counts of first-degree murder, aggravated manslaughter, and aggravated child abuse, brought out excruciating detail from Garwacki of the abuse 3-year-old John Taylor Baxley suffered at O'Connell's hands. Pushed to acknowledge his contributions to the numerous bruises found on Baxley's body, Garwacki allowed: "I wasn't a good father."Even for a man given to monosyllabic responses, this was an understatement.Between his father and O'Connell, both of whom thought the toddler might be intellectually impaired, the boy was slapped, whacked, punched, and bitten, said Assistant State Attorney Rita Peters. These punishments were dealt out as discipline for wetting the bed or not eating his meals. His lip was split, his left arm was broken, and there was an open wound on his forehead. He was never taken to a doctor."I witnessed (O'Connell) pick him up by the throat, hold him up to the ceiling fan, and tell him if he didn't get s--- right, she was going to hang him from the ceiling fan," Garwacki testified.The boy's crime: imperfectly recalling his ABCs.Another time, Garwacki said, the toddler dropped a 3-pound weight he'd been made to hold by his broken arm. "She choked him long enough to where his body went limp," he said.According to prosecutors, all of these injuries occurred in the span of the roughly six weeks Baxley spent in Tampa with his father, who had only met the boy once before and rarely spoke to him. It was a summer trip Baxley's mother had grudgingly approved — they were divorced with joint custody — and about which she had grave doubts.On Aug. 10, 2010, Garwacki said he returned home from a trip to buy the narcotic Roxycodone, which he and O'Connell both used, to find his son lying unresponsive on the bathroom floor and O'Connell in shock. Within six months, prosecutors charged both with the boy's murder.What ultimately killed the toddler, according to the state, was "thymic involution," a crippling of the immune system that some researchers have diagnosed in physically abused children."What it basically means is he stopped fighting," Peters said. "His body could not fight off the injuries and he finally died, he gave up."O'Connell attorney David Knox said the cause of death was much less firm than prosecutors made it appear."The state does not know how JT Baxley died," Knox said, using the boy's nickname. It's possible Baxley died from a seizure, or a heart arrhythmia, or his untreated broken arm, he said.Seeking to portray Garwacki as the boy's primary tormenter, defense attorneys for O'Connell held out the plea agreement he had reached with the state as proof that he had reason to shift the bulk of the responsibility to their client. They pointed to the different accounts he gave to police, which diminished both his and O'Connell's role in the abuse."Do you think you killed your son?" defense attorney Anthony Rickman asked Garwacki, who has already pleaded guilty to second-degree murder."No," Garwacki replied.