A scenario (suspend your disbelief about specifics) and a question.
OP had a bad few days. Reeva comes over totally into VDay, OP not so much. Not a great night and at some point they fight. A lot. OP goes to sleep. Reeva can't. She goes downstairs and gets a snack, then comes back upstairs. OP wakes up and is angry all over again. They fight again, this time more intensely. Reeva considers leaving, but hasn't decided for sure. Both are tense. OP gets up to do whatever.. move fans, go on balcony, go downstairs... whatever. ( Yes, I know what he said he did, but since y'all think he lied about everything, why believe what he said he did here?)
Continuing. Reeve slips out of bed and goes to the loo. She may have decided to leave, or maybe she just had to pee. In either case she's quiet on purpose because she doesn't want the fight to escalate any further.
OP hears a noise in the bathroom. He is already enraged, has not let go of the fight. He doesn't check for Reeva. His first thought isn't protecting her, its that a SOB might be in his house.
He's angry - very - but he also is feeling vulnerable. Not afraid, but vulnerable because he is at a physical disadvantage. Just reality of the matter, and part of the reason he feels compelled to compensate for that disadvantage by always reaching for a gun.
The thought of pushing the panic button or running the other way never occurs to him. It doesn't because response to the panic button isn't instaneous, because if he turned his back to flee he might be overtaken, because he is OP, who isn't going to back down, and because his reasoning is already impaired by rage, increased exponentially by his perception that an intruder dared to threaten him.
He grabs his gun, runs down the hallway screaming get the f out. Reeva has no idea what's going on and stays silent. OP hears her move, perhaps she reaches for the door handle. He doesnt fire a warning shot because it never occurs to him to do so, not because he's afraid of a richocet.
He shoots once. The noise is deafening. Literally.
He doesn't hear her scream because of the deafening noise. He is also literally pumped on adrenaline, and furious. He pauses for a brief moment because the noise hurts his ears, and because the temporary loss of hearing increases his anger and his fear. He fires 3 more shots. He fires them knowing that he is firing them, and yes, hoping that the shots will eliminate the threat. He isn't trying to kill whoever is behind the door, but he knows enough even in that mental state that he is very likely to cause them serious injury.
Given that scenario ......(please don't argue about the details...its only a scenario) ....do you think he should be found guilty of murder?