What makes Winston turn on Julia toward the end of the story? Explain the pinnacle scene where this happens.
I think that what you are talking about is what happens to Winston in Room 101 at the end of Part 3, Chapter 5. In this scene, Winston is subjected to the worst torture of all (and the last torture that we see happening to him).
O'Brien knows that rats are Winston's greatest fear. So, for this climactic torture, he puts a mask on Winston that has a rat cage strapped to it. The rats will be able to get in and eat at Winston's face.
When threatened with this, he starts begging O'Brien to do this to Julia, not to him.
At that point he has turned on Julia and he does it out of fear.