Hi beach, thanks for the thread.
I have two theories. One is based on what the evidence has and has not shown, and the other is based on conjecture of what might have happened.
Theory #1: Things happened essentially as OP has described. He believed there was an intruder in his bathroom and believed Reeva was in bed when he shot through the bathroom door. He may have embellished or polished his testimony to try to paint himself in what he thinks is a better light, legally, and he may be hiding some things but on the whole - this was not an intentional murder of Reeva and there was a mistake involved with his perceptions. This is what I think the evidence supports so far.
Theory #2 - Something caused Oscar to be fed up with the relationship that night and he decided impetuously to end it - much like when he told Samantha Taylor it was over and she needs to take his car and her things and leave.
I don't think it would have been a jealous rage, but more like OP just not being able to cope with the relationship and acting impetuous in the moment and telling Reeva to leave. Speculating, and trying to fit the evidence into this theory - perhaps Reeva disarmed the alarm and went down for a midnight snack and in the process woke OP up, which annoyed him. He might have thought Reeva was being inconsiderate of his need for sleep since he is a top athlete and must have the right food at the right time and the proper amount of sleep. We've seen this before from OP in the text messages - he expects that his needs will be accommodated.
Reeva, being a more mature and self assertive woman than Oscar was accustomed to, might have retorted that there are two people in this relationship and that she has needs as well and it's unreasonable for him to get irritated at her getting a late night snack.
Oscar would not be expecting such a response - in the past when he griped about something and put his foot down, Samantha Taylor begged his forgiveness and let him have his way. So perhaps Reeva's more rational and mature response threw OP into unfamiliar and uncomfortable territory that he was not emotionally or intellectually equipped to deal with. Given his impetuous nature and inability to cope with discord, he might have become overwhelmed and began to demand that Reeva just leave because he didn't want to deal with the relationship and could not just tolerate the tension for the remainder of the night.
I could imagine that in response, Reeva may have laughed or told him he was being absurd and immature to react in such an over the top manner. This would only increase OP's anxiety and frustration - feeling increasingly helpless to control his environment and unable to bear the increasing tension of the situation. (Remember that he tends to just go away and not deal with things when there is discord - e.g., dropping Reeva at home when they were in a tiff instead of having her come to spend the night; telling ST to leave and go away when he was frustrated that she was defending the old guy who whisked her overseas, etc).
With his frustration and anxiety rising to a pitch, he shouted at her to "GET THE F*CK OUT OF MY HOUSE!" and threatening in a loud voice "I'm calling the police if you don't get the f out of my house." At this point, Reeva became alarmed at his overreaction and ran to the toilet with her phone and his phone to keep him from calling the police and escalating the situation into a really bad situation for everyone. Perhaps she just wanted to give him a moment to calm down, but instead it had the opposite effect on him. He became more frustrated, more emotional, and more out of control of his emotions. He yelled again "Get the F out of my house" and when she didn't reply, it threw him over the top and he grabbed his gun and shot - basically out of frustration and impotence before really thinking of the consequences. Then he realized what he had done and freaked out and yelled and screamed and cried and so forth.
There is not sufficient evidence to support this, but nevertheless it would be a possible scenario that would fit with the dynamics of OP's personality and his way of reacting to emotional events.