In my mind there is no way he would he would have shot 4 times thru a closed door without confirming who was in the bathroom (a stranger or Reeva).
I think he's an arrogant asshole and a ticking bomb, they had a row, she shut herself in the bathroom or else fled there in fear of her life and he shot her in a rage because she wouldnt comply with his wishes. He probably didnt intend to kill her at any point until he saw red and pulled the trigger, then had immediate regret at what he had done.
He can regret it all he wants, he knew it was her on the other side of the door and killed her and at the moment he pulled the trigger he wanted her dead.
That is my take on it.