Here is why I think David's flip on Kristina was a good move for David. It might not have been the best move for Ovambo, but it was a good move for David.
Just to re-visit that vote, it was a 4-4 split with Stephanie/Kristina/David/Mike on one side, and Ashley/Sophie/Whitney/Julie on the other. Mike won immunity and Ashley played the idol. The vote was between Kristina and Sophie. In rocks, the only people who could've pulled rocks were David, Stephanie, Julie or Whitney. 50% chance an Ovambo pulls it, 50% chance a Nama pulls it. Whitney/Julie, how do I put this in the most polite way, are not the biggest challenge threats nor threats to find the idols, so having them in the game is not the worst thing in the world for David. David is in an alliance with Stephanie (and Mike), while Kristina has a working relationship with the Nama girls (pre-Grant boot), so there is reason to argue that she COULD, in his mind, flip to further herself. So, David might've thought, I don't want to go home. I don't want Stephanie to go home. If I'm losing someone from my side, I'd prefer it to be Kristina.
Now, that being said. I don't know how David/Stephanie/Mike feel about the alliance I had with the 3 of them, but if I was in any of their shoes, I was always tighter with those 3 than anyone else in the game, and anyone else in the alliance. David knows if Kristina goes, he, Stephanie and Mike are moving forward. He did have a numbers advantage - yes, it SHOULD have been incredibly stupid. But it worked out for him. Whitney didn't vote, so the following vote split 3-3. It shouldn't have worked out, but it did. I don't know the full of it, obviously, I can only see what people say at TC but I do not have any reason to believe that David FLIPPED on Ovambo, just simply hedged his bets to move himself a step further. There were 2 favorable outcomes, there were 2 unfavorable outcomes. He hedged his bets and took the middle option. He proceeded in the game with Mike and Stephanie and they appear to be rock solid with each other, as I expected. I would have been shocked at that point forward if any of these 3 flipped on each other.
Whitney not voting sort of forgave David of his "flipping". I don't like calling it that because is it really flipping? really? JULIE flipped. David didn't. If they go to rocks at that point and Whitney or Julie go home, sure, it worked out in David, Mike, Stephanie and Kristina's benefit.
I don't think it was such a horrific move. I don't think he is a goat. I don't think Mike is a goat. I don't think there are any goats left in the game. Just my two cents. I still believe Ashley is the one to beat, even though she is on the wrong side of the numbers. I think if the Ovambo-3 don't vote out Ashley and Sophie and go to the final 3 with each other, it would be very very dumb of them.