BIB - we can only speculate. However, his reckless behaviour had been escalating in the months before the killing, and he was somehow always permitted to get away with it. Almost a perk of being such an icon. On the night Reeva was killed, it's not hard for me to believe he saw red when she wouldn't come out of the toilet and things just happened from his rage. He didn't know if she was calling the police, so if he was screaming at her to come out and she was yelling back (if she was) you can just imagine how mad he'd have been. First, her not obeying his instructions, and secondly, the possibility she was calling the police to report him for violence. Some people, when they are in a red mist, will do the most extreme thing to gain back control of the situation.
I think the fact OP hadn't shot anyone up to that date was because he hadn't been in that specific situation before. Once he was in it, I think his rage took over and all he could think about was getting Reeva out of the toilet and away from the phone. The smashing of the bat on the toilet door (rage), and the final bullets put him back in control and kept whatever Reeva might have been going to tell the police, secret. I can totally see him killing her in order to protect himself and his image. He's a cold unfeeling human being (shoots dogs and drives off without a word to the dog's owner who's standing there...) - so the 'why' for me is more about 'why not'. He did what he did to protect himself, just like the Tasha's incident when he got someone else to take the blame as he didn't want negative publicity. Maintaining his image was more important to him than Reeva was.
Great points - and re Bold text
This is the way DV always goes.
The cycle continues, perhaps over years but escalating until the abuser does something that cannot be covered up.
Usually in a country like NZ (few gun murders), the signal abuse will be GBH that the women will survive. Kids may not be so fortunate.
The point though is that the first time this comes to light will usually appear to be "the first time"
My speculation
1. OP has a long history of abuse - escalating to this assault. However Reeva takes the priors to her grave and only one other has talked.
2. Numerous abusive incidents are documented. e.g the drunken gun incident at home. The behaviour etc etc
3. As you point out - he was in a cycle of reckless escalation - Tasha's, the sunroof, the boot incident. Had he shot someone at Tasha's then that would have been his moment
4. His association with a macho/criminal gun toting element
5. The background problems in their relationship (see the work by J13)