Arias, 32, charged with murder in the 2008 death of Alexander in his Mesa, Ariz., home, was expected to be cross-examined beginning Thursday by prosecutor Juan Martinez, who is expected to go after Arias' self-defense claim, ABC News reported.
Arias told the jury Wednesday on direct examination that Alexander became enraged when she dropped his new camera as they were taking nude photos in his shower. He picked her up and slammed her onto the floor.
Arias said she fled to his closet to hide but could hear Alexander's footsteps following her. She grabbed a gun from his shelf and pointed it at him, hoping he would stop, but he didn't.
"He got like a linebacker. He got low and grabbed my waist, and as he was lunging at me the gun went off. I didn't mean to shoot. I didn't even think I was holding the trigger," she testified.
Asked by her lawyer if she were convinced Alexander intended to kill her, Arias answered, "For sure. He'd almost killed me once before and now he's saying he was going to."
Arias earlier testified that Alexander had once choked her.
She told the court that she doesn't remember stabbing Alexander whose body was found with 27 stab wounds, a slit throat and two bullet wounds in his head.
"I have no memory of stabbing him," she said. "There's a huge gap. I don't know if I blacked out or what, but there's a huge gap."
After lying for nearly two years, she eventually confessed to killing her ex-boyfriend, but insisted it was self-defense, ABC reported.
"The main reason [for lying] is because I was very ashamed of what happened. It's not something I ever imagined doing. It's not the kind of person I was. It was just shameful," she said on the stand Wednesday. "I was also very scared of what might happen. I didn't want my family to know that I had done that, and I just couldn't bring myself to say that I did that."
Arias said she also kept lying because she was planning to kill herself before trial.
"I was concerned with how it would affect my family. I wanted to die. I was going to definitely kill myself," she said. "That was my plan."