What a reasonable person believes....There's the rub. I live in an American city that routinely ranks in the top 10 for number of murders and violent crime. Home invasions during the day are fairly common in my neighborhood, as are break ins, car thefts, etc. Police sirens and strobe lights from police helicopters chasing down thugs are also routine. (BTW, I live in a middle class neighborhood mostly populated with academics, attorneys, and political progressives).
Because of where I live I can believe and accept that OP feared intruders, regardless of his gated community and security systems. His fear is made more believable by the fact that he has a history of hearing sounds in his house and leaping to the conclusion the sounds were made by intruders.
IMO, if he wanted to spin the best possible lie, he would have said that Reeva did not wake up when he got up to bring fans in, etc. Having her be asleep would make it more believable that he thought she was still in the bedroom when he heard a noise in the bathroom.
Jumping from point to point, but they are all linked.... I don't think OP was especially in love with Reeva. I think that they may well have argued that night, and that in any case there was little genuine intimacy that night that bound them together. That matters, because I think emotional detachment readily explains how it could be that OP heard a noise in the bathroom and zeroed in on the perceived threat without reassuring Reeva or making contact with her.
Shooting into a closed door. I no longer believe OP thought Reeva was behind that door, nor do I believe OP that he shot to protect Reeva. I think that OP freaked and that shooting a gun in response came naturally to him. In OP's mind the threat was real and immediate and shooting at it was reasonable.
OP didn't treat Reeva well, IMO. But it is equally true that there is no evidence whatsoever that he was physically abusive to Reeva or to any other woman he was ever involved with. He had everything to lose and nothing to gain by hurting Reeva, and no pattern of behavior that would indicate an escalation to extreme violence, much less murder.
He screamed for help minutes after shots were fired, and called for help as well. Not the actions of a man trying to hide anything at the time.
His unwillingness to accept full responsibility for killing Reeva may do him in. However revolting that unwillingness is, though, doesn't mean he is guilty of premeditated murder.