I suspect Nathan didn't care about her deceit because he was talking to David about his failings. If he had mentioned that she failed too, then the message to David would have been greatly watered down. David was completely responsible for what happened. As a man, and as the king, the responsibility was with him. No one else.
Bathsheba on the other hand was responsible for her actions. We know that she decided to be naked in full view of the king while her husband was away. That doesn't sound accidental to me. David was THE premier alpha male of his time. I'm sure most all women wanted him. I'm sure most all men wanted to be him. The fact that Bathsheba actually succeeded doesn't make her any worse really than all the rest. I'm sure others tried similar things. And I'm sure many, many others wanted to but couldn't for one reason or another. You're responding like I'm ripping on her. I'm not. She was just human, like the rest of us. I kinda like her actually.