Raleigh, N.C. — Defense attorneys trying to keep an Iraq war veteran off death row argued Tuesday that his undiagnosed post-traumatic stress disorder – combined with prescription painkillers and alcohol – incapacitated him to the point that he was mentally incapable of planning and carrying out the alleged sexual assault and beating death of his 10-month-old stepdaughter.

"He's got to pay for what he did, but what we're asking you is to not make him pay for what he didn't do," attorney Terry Alford told jurors during closing arguments of 28-year-old Joshua Andrew Stepp's first-degree murder trial.

Stepp, Alford said, is guilty of second-degree murder in Cheyenne Yarley's Nov. 8, 2009, death – something Stepp does not deny.

Testifying on his behalf, Stepp admitted that he beat, shook and slammed Cheyenne into the carpet of their Raleigh apartment when she wouldn't stop crying. But he didn't know why he did it, he said.

"He felt something was going on with his brain that day," Alford told jurors, adding that his client was trying to self-medicate in any way he could.

"Josh was on a mission to get drunk, and he can't explain it," Alford said. "That makes a lot more sense than 'I think that when I get home, I'm going to rape my 10-month-old child, and in doing so, I'm going to hurt and injure her so bad.'"

But Stepp did not sexually abuse the girl, Alford said.

Injuries to Cheyenne's body were consistent with sexual assault, state witnesses testified during Stepp's two-week trial, but they could not say for sure what caused them. Although the child's blood was found on Stepp's underwear, Cheyenne's DNA wasn't found on Stepp's body.

The injuries, Alford said, happened as Stepp, unable to control his frustration of the crying infant, changed her diaper several times that night – injuries "consistent with harsh cleaning, using a finger and wipes in an overly aggressive way."

Wake County Assistant District Attorney Boz Zellinger argued, however, that Stepp was angry because he had to leave a local sports bar to take care of Cheyenne while her mother worked. He knew exactly what he was doing and tried to rape her and then cover up the crime, Zellinger said.

"Men and women come back form wars afflicted with PTSD. This defense taints their suffering," Zellinger said. "This defense perverts that disease. Having PTSD does not make you rape and sexually assault a 10-month-old."

Stepp's cognitive abilities were not affected that night, Zellinger said, and his ability to remember insignificant details while forgetting important events was somewhat of a convenience for him.

"He remembers making a decision to wet the toilet paper (that he put in her mouth to try to get her quiet), but he doesn't remember punching Cheyenne in the head, and he doesn't remember how he got her blood inside his underwear," Zellinger said.

It's unreasonable, he added, to think that Cheyenne's blood could have gotten on his underwear any way other than a sexual assault.

"There's no defense to the sex offense. There's no defense to felony murder by that sex offense, and as to everything else, the defense is weak in that the evidence is overwhelming," Zellinger said. "The only thing that can lead you away from that evidence is the defendant's word – and that's a man who lied repeatedly that night while his stepdaughter was dying."

Stepp told his wife that the child fell off the couch and hurt her face, Zellinger reminded jurors, and told a 911 dispatcher, firefighters and police officers that she choked on toilet paper.

"This assault wasn't a one-second thing and then 911 was called," he said. "This assault took place over an hour and a half, a torturous hour and a half, for this 10-month-old child to live through –that she didn't live through."

Jurors are expected to begin deliberating Wednesday.