When they get back to Narnia, Edmund doesn't want to be there anymore: "let's get out, they've gone." It holds no joy for him, now that he is on the wrong side.
Peter shows his character in being quick to apologize, and to explore.
Edmund slips and lets everyone know he lied about not being here before. But he doesn't apologize, choosing rather to resent them all inside.
Lucy takes them to Tumnus, but he's been arrested by the White Witch. Lucy fills them in. Susan wants to go home now, for safety. But Lucy knows loyalty; she is responsible and they have to help him for doing the right thing.
At this point they are kept from pursuing the witch, who they couldn't stop if they tried, but are taken to the beavers providentially.
On the way, Edmund plants doubts in Peter's mind. What if good is evil, and evil is good? How do we KNOW which side is right? But deep down, already when he met the witch he knew she was bad. He has descended to leading his siblings astray out of spite, and self-justification.