Okay, I'll take a crack at it, although my theories won't be completely fleshed out since I have not seen all the evidence & missed some of the earwitness testimony.
Theory 1 - OP is telling the truth
During the course of the evening, they argued over something or several somethings. OP was in a heightened state already upon arriving home and it went from there. He'd gotten bad news that day and was feeling out of control which manifested into anger and distraction.
At 10pm, RS did yoga, he surfed the net. They likely had sex but it wasn't fulfilling for RS. She'd been doubting if this relationship was really working for almost two weeks and given his mood, the bad sex, the hot room and that she couldn't sleep, she slipped downstairs to grab a cold drink and had a small bite to eat out of the fridge. OP was sound asleep & didn't hear her. She slipped back upstairs, got back in bed & tossed/turned until a little before 3am. She got up and snuck to the bathroom, having grabbed her overnight bag. Very quietly, she packs it and starts to get dressed. Realizing her skinny jeans are going to make noise against her skin, she opens the bathroom window and tosses them out to retrieve them & put them on once downstairs. She really doesn't want to wake OP & start arguing.
OP hears window open, and the rest is his version. The reason RS never said a peep was because of her past experience with a home invasion. However, she did scream when he shot her, but OP couldn't hear her screams, as his ears were ringing.
OP is still lying about certain details (e.g. "can't you sleep, baba?", hearing wood move in the bathroom, etc.) because he really is petrified of going to jail or simply thinks he can beat this. Although more concerned with himself, he does feel genuine remorse and his PDEs are part due to nerves/part due to real remorse and being reminded/confronted with the details of how horrific her death really was.
Theory 2 - OP killed her in a rage
OP comes home in a heightened state. He'd received horrendous news that day, felt out of control and belittled. This was manifesting into seething anger. He snapped at RS about the dogs the minute he walked through the door. RS tried to sooth him which did the opposite and he pushed her away. She felt in her gut she should leave, but since it was the day before VD and she was trying to make this work, she stayed. She gave him his present and tells him that he can open it now if he wants. He tosses it on the table and dismisses it, saying it isn't even VD yet. RS's feelings are hurt, so she goes to make him dinner (swallowing tears, but keeping composed) while he runs upstairs to change. He really doesn't want her there, plops down on his bed and watches *advertiser censored* (to take his mind off the day). RS calls him down to dinner where he once again is moody, sullen, non-affectionate and doesn't even thank her for the meal - may have even been critical. RS starts to cry which only makes him madder.
From there, I simply don't know and can't begin to fill in the blanks. But you've got the following:
OP -
- 26 year old male with a very short temper
- moody and critical, snaps easily
- reckless and impulsive
- a celebrity used to getting his way
- not very bright and certainly not self aware
- got some damaging news that day, likely about his career
- his home (possibly territorial)
- his gun
RS
- 29 year old female
- very intelligent
- on the verge of real career success
- positive and happy
- nurturing & giving, possibly to a fault
- assertive
- likely high maintenance with concrete expectations
I think there was sex and arguing, no idea what order, but likely after sex was when RS texted that she was staying the night. This would have been around 10:30. I think they did get into bed together, looking at cars, but for some reason the arguing started back up again. OP was getting crueler in his comments and was angrier than she'd ever seen him. To try and diffuse the situation, she goes downstairs to make herself a drink, snacks out of the fridge (nervous eating). Then OP comes down not willing to be dismissed or ignored, more arguing. RS runs upstairs to get her things and leave. She was likely crying and now stressed and locks OP out of his bedroom.
And then escalation, likely shoving, certainly threatening, but by the time RS ended up in that toilet closet, OP had gone from heightened state to murderous rage, firing into the door knowing full well who his bullets were intended to hit.
I think RS was beyond hysterical not only because of OP's behavior but due to her past home invasion experience. There she was feeling trapped and scared out of her wits again. Her hysteria would have infuriated OP, especially in his home. I base that on how angry he got at the girl he kicked out due to drunken behavior. Only in that instant, he snapped and slammed a door in front of her. With Reeva, it was a gun.
I bet OP lacks self control when he feels his most vulnerable so the arguments they were having must have tapped into his tiny little bruised ego. He lacks the insight and humility to ever think anything is his fault, so it doesn't surprise me that he could quickly come up with the intruder story. Plus, saying he mistook her for an intruder is really the only defense he could come up with given the circumstances.
But that's all I got at this point. I do think Version 2 is more likely the truth and I base this on OP's testimony. His version isn't credible or logical for too many reasons to list, plus all the changes and additions to it. When describing a banana and it looks, smells and tastes like a banana, it's a banana. OP's version looks, smells and tastes like sour grapes to me.