Gwen doesn't know why she turns away. Merlin's her best friend and she can see in his eyes that there's sadness at her leaving but she… she can't. She just can't.
Because Merlin may be her best friend, but he's also Arthur's (whether anyone wants to admit it) and she just can't take one more person who thinks she's an utter failure and a terrible person. She can't cope with him asking why because she doesn't know. It all seems a blur to her now, all she remembers is those feelings, the feelings she'd had three years ago (three years) coming rushing back stronger than ever. She was in love with Lancelot but she doesn't remember it being a burning passion and the feeling that if she didn't touch him then she wouldn't be able to go on. She doesn't understand how even if that was how she felt she could have done this to Arthur.
She doesn't want to look into Merlin's eyes and see disappointment. She tells herself it's because she can't bear to make him feel torn between herself and Arthur, but it's so much more selfish than that. She can't bear it because she knows who he'll pick. It's as it should be of course; she is the one in the wrong. But she'll have her entire life to blame herself and right now all she has to do is get out of Camelot without breaking down.
Merlin knows why she turns away. It's strange, because Gwen's always been there; she's always been his friend, constant, well-meaning (he can't shake the feeling that there was something darker at play than reawakened emotions but he can't see what and he certainly can't prove it); he doesn't know if he can face Camelot without her.
He can't believe that everything was so happy just a few days earlier. The jousting tournament thrown in her honour and her looking every inch the queen as she smiled and applauded the jousters. The people had loved her. It's sad, really. She would have made an incredible Queen.
He can't condone what she did, but neither can he condemn it. Exile – whilst in some ways merciful – seems to him to harsh a punishment even as he understands why Arthur's doing it.
But as he watches her walk away, he can't shake the feeling that this isn't the end. It shouldn't be the end. It couldn't be.
Because he knows, in the bone-deep way he does know about these things sometimes, that Albion cannot be born without Gwen. And – further than that – he knows that Arthur will never be happy without her. Not truly.
He doesn't think he will be either.