So Briony was jealous of Robbie that he wouldn't give her attention..

Sure she knew about sex but she didn't understand it at all. Remember her talk about her play: "Love is all well and good, but you have to be sensible."
I don't think her jealousy turned to hate, not in the one afternoon of the time frame in which this all took place. Briony was quite fond of Robbie at the beginning, "Will you come see my play?" she asked him, looking nervously hopeful.
Certainly, his rejection of her after she jumped in the river a few years earlier was a factor but it was a confluence of events happening at a particular time in her life that helped her accuse Robbie.
And because the story is entirely Briony's, we don't know what the real reason was. She's about the most unreliable narrator there could be. We don't know that Robbie was remembering Briony and the river jump as he was traipsing through France. Briony put that thought in his head to give the reader a sense of motivation. I think McEwan, the real author, was showing us what a jumble it all was in Briony's head.

Yes she was jealous, but that is no excuse for her behavior.

On the other hand, if Robbie and Cee had a better sense of decorum, and had gone into a room for their tryst (instead of banging against the bookcase in the library), an impressionable 13 year old girl would not have seen it, and been traumatized by it. So there was bad behavior all around.









Schrodinger's cat walks into a bar and doesn't.