During the second day of gruelling questioning, prosecutor Gerrie Nel began to pick at Pistorius's explanation of why he fired four times through his toilet door on Valentine's Day 2013.
Pistorius has been charged with murdering his 29-year-old model girlfriend and faces a life sentence if convicted.
The Olympian has said he fired the shots accidentally and did not mean to kill anyone. He testified that he feared someone was coming through the toilet door and that his life was in danger.
"We know for a fact there were no intruders in your house that night, we know for a fact there was no ladder against the wall," Nel said.
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"We know for a fact that you had no reason to shoot, objectively speaking."
Pistorius responded: "That's correct my lady."
Regardless of who Pistorius believed was behind the door, he could face a stiff sentence if Judge Thokozile Masipa believes he purposely used lethal force without reasonable cause.
During the five-week trial the world-famous athlete has appeared fragile, frequently crying in court and becoming physically sick when the gruesome details of Steenkamp's injuries were discussed.
Nel spent most the first part of his cross-examination accusing Pistorius of being a selfish, controlling boyfriend and caring more about himself than the death of his girlfriend.
Nel tore into the 27-year-old's account of his relationship with Steenkamp.
"It's all about 'I'. It's all about Mr Pistorius," Nel said, reading cell phone messages in which Steenkamp said she was upset and "scared" of Pistorius's behaviour.
Nel also accused Pistorius of making a public apology to Steenkamp's parents just to make himself feel better.