Maybe that's because he isn't a sociopath or maybe even a narcissist.
I believe he had a rash of anger which he couldn't control. Anger which had escalated during whatever argument they were having. Before he knew it, it was done. It had gone too far. It's still pre-meditated, IMO, b/c even though he was in a rash of anger, he still picked up the gun, walked to the door, and shot her.
I don't see him at all as a Jodi Arias type or a Scott Peterson type, etc.. Or Martin MacNeill, or countless others like that.
I do believe he is sorry for this whole situation. And if he could do it all over again, he would do things much differently and Reeva would still be alive.
The difference b/w him and the others like what you mentioned, is that the only thing those types regret is that they got caught. For example, they would still have liked for the other person to be dead, just maybe in some other way so that they wouldn't have gotten caught. YKWIM?
I believe Oscar does not want Reeva to be dead. He wishes he could hit a "rewind" button and have the whole thing never happen.
So that is the difference, IMO.
But that doesn't mean that Oscar's fist priority right now is not Oscar. In fact, that is what makes most sense to me. He is in a fight for his freedom.
When he steps into the courtroom every day, IMO, that is his focus and that is his goal. I don't think he has that much energy/concentration/focus right now to be so incredibly focused on both Reeva and himself.
Meaning - yes, I believe he has cried a lot about this in private. Wishing it had never happened. Wishing he had just make different choices that night.
But when he comes into the courtroom - his whole focus is on himself. And in the recent months leading up to court, he has probably been practicing his "lines," practicing his "story," etc..
I don't think it's genuine what he's showing in court. I think he has thought about a certain image he wants to portray to the judge. He wants to portray that he is weak, that he is scared, that he has become so emotionally-distraught by all of this that he cannot possibly think of anything else and cannot possibly formulate any story or lines or lies. That he is just barely stringing himself along.
That's why the crying, comiting, etc., suddenly comes on during certain CUE moments, like discussion of Reeva's dead body, discussion of first finding her after he killed her, etc..
JMO.