The court in Pretoria, South Africa has heard how the Paralympian fired four shots in quick succession after hearing someone inside the bathroom.
Speaking under cross-examination, the 27-year-old said: "When I heard a noise, I didn't have time to think and I fired my weapon. It was an accident.
"If Reeva had come out or she had spoken to me I wouldn't have fired. The noise coming from the bathroom made me pull the trigger."
Prosecutor Gerrie Nel went on to read from Reeva's text messages, one of which said: "I've been upset with you for two days now... I'm scared of you sometimes."
He added: "Why would she be scared of you?"
Pistorius replied: "I think she's scared of the feelings that she had for me, she says 'I'm scared of you sometimes and how you snap at me and how you will react to me'.
"I never shouted or screamed at her. It hurt her feelings about the way I would react."
The lawyer went on to accuse the athlete of "picking on" the model, claiming Pistorius "did not care enough" to tell her he loved her.
He also asked why the athlete had yet to apologise personally to Reeva's family, to which the athlete replied that he was "terribly sorry".
"I am terribly sorry that I took the life of their daughter. I didn't think they would want to see me."
Pistorius has pleaded 'not guilty' to the premeditated murder of his girlfriend and the illegal possession of ammunition.
The case continues.