The court in Pretoria heard the athlete fired four shots in quick succession after hearing someone inside the bathroom.
Fighting back tears, 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."
Pistorius had been in a relationship with Reeva Steenkamp for four months
During a second day of cross-examination, Pistorius was questioned about text messages sent by Miss Steenkamp which claimed she was "scared" during their four-month relationship.
Prosecutor Gerrie Nel read from one message 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."
Text messages between the couple have revealed arguments
Mr Nel accused the athlete of "picking on" Miss Steenkamp, claimed Pistorius "did not care enough" to tell Miss Steenkamp he loved her and insisted their relationship was focused on "me, Oscar".
Pistorius told the court in Pretoria he was "terribly sorry" he had taken his girlfriend's life after he was asked why he had not apologised in person to her family.He said: "I am terribly sorry that I took the life of their daughter. I didn't think they would want to see me."
Pistorius was accused of blaming his legal team for not questioning claims made by witnesses and was asked a series of questions over his handling of firearms and ammunition.
He has admitted a handgun went off in a restaurant while in his possession, and told the court he carried his gun everywhere and usually had a bullet in the chamber.
"The weight is light. My gun has a double safety. I usually carry one up," he said.
The court heard Pistorius had ammunition belonging to his father in the safe at his home, for which he did not have licence. He also had an extra magazine in his bedside draw which was found there when he was arrested and usually carried his gun with him at all times for safety reasons.
Pistorius denies premeditated murder and illegally possessing ammunition and two further counts related to shooting a gun in public in separate incidents prior to the killing.
