It is one of several possibilites that can be listed, which might be enumerated as follows.
1. He knew Reeva was in the cubicle and decided to kill her so no one else could have her.
2. He knew Reeva was in the cubicle and shot recklessly in anger, act in haste and repent at leisure.
3. He knew Reeva was in the cubicle and shot at an oblique angle in anger, thinking come out of there you bitch.
4. He knew Reeva was in bed asleep, and a black man had broken and entered. (this is from the LRB dissertation, literally it is always a black man before I am vested with racist accusations
http://www.theguardian.com/world/2015/dec/03/oscar-pistorius-appeal-murder-jacqueline-rose) Therefore he knows his life is in danger and shoots to kill in his and Reeva's defence.
5. He knew Reeva was in bed asleep, and a black man had broken and entered. He shoots at an oblique angle, imagining the intruder will not receive an injury, but be totally aware that he is nailed down by a serious and prepared defender.
I believe 5, but as I concede, it ain't getting traction.