Pistorius, a former Olympian, has admitted he shot Steenkamp but claims that he mistook her for a burglar in the bathroom of his residence on February 14, 2013.
Pistorius testified that he was "overcome with a sense of terror and vulnerability" when he suspected an intruder was in his bathroom.
"I didn't think about pulling the trigger," Pistorius told the court on Tuesday. "As soon as I heard the noise, before I could think, I pulled the trigger."
The trial is expected to continue until mid-May, at which point the judge -- under the South African judicial system -- will consult with two experts in order to decide the verdict.