Raleigh, N.C. — An Iraq war veteran facing a possible death sentence if convicted in the beating death of his 10-month-old stepdaughter testified on his own behalf Tuesday, saying he never meant to kill the child and that he only wanted her to stop crying.

But an emotional Joshua Stepp said he couldn't explain why he beat, shook and slammed Cheyenne Yarley's face to the carpet of their Raleigh apartment while her mother was at work on the night of Nov. 8, 2009.

"I can only remember the really intense parts – when I was hurting Cheyenne," he said. "I don't know why I can remember that."

Defense attorneys say that Stepp, 28, was suffering from post-traumatic stress disorder from having to pick up body parts of fellow soldiers after they were killed in a roadside bomb in Iraq.

He had been self-medicating with alcohol and prescription drugs, which, combined with Cheyenne's nonstop crying that night, had all the makings of a perfect storm, the attorneys say.

Stepp admitted to drinking heavily on the day of Cheyenne's death and said he had broken open four prescription painkillers to feel the effects from them more quickly.

"I felt like I was on some sort of mission to get messed up," he said.

The infant would always cry when her mother left her, Stepp testified, and he thought that she would eventually calm down, like she always would do.

But she never did.

"It's maddening. I'm trying to do what I can to stop her crying and just nothing is working," Stepp recalled. "It's frustrating. It's confusing. It's like you can't fix something. It's right there in front of you, but you can't fix it."

Prosecutors contend that he not only beat the infant for nearly an hour but that he also sexually assaulted her. The infant showed signs of sexual abuse, an emergency room doctor testified last week, and investigators found her blood on Stepp's underwear.

Defense attorneys say that Cheyenne was injured when a frustrated Stepp changed her diaper repeatedly that night. There was never any indication he had sexually abused her before, they say, and investigators never found any of the girl's DNA on Stepp.

Stepp, who's expected to continue his testimony Wednesday, denied the accusation Tuesday when his attorney, Terry Alford, asked him about it.

"No. God, no," he said, but said he didn't know how her blood ended up on his underwear.