If anyone else had lain with the Queen, Arthur would not have hesitated to have them killed. But this is different. This is *Lancelot*. And Arthur is reluctant, not least because he knows that Agravayne, and through him, Morgause, are involved in all this.
Lancelot is his most trusted knight, his closest boyhood friend. And Arthur cannot find it in his heart to condemn this betrayal, although it breaks his heart twice over.
So when Lancelot comes to pull Guinevere from the fire and carry her away, as he will - as he must - Arthur will let them go.