More about 3x01: Although Beckett "punishes" Castle for the hurt he did to her by leaving with Gina and the consequent summer of silence, do you think that, on some level, she knew that she was also partly to blame for what happened at the end of 2x24?
Well, yeah. Espo tells her in 2x24 that Castle's leaving because he isn't gonna stick around and watch her date someone else. She kind of blows that off and doesn't really say anything in response, but it gets her thinking enough that she acts on what she was already feeling -- that as nice as Demming is, he's just not what she wants. And you see it in her face when he asks about Demming in 3x01. She doesn't want to tell him they broke up. She basically broke up with the guy because he wasn't who she wanted, she's been single for months by this point; the hurt in her eyes isn't because she misses Demming. It's because she's embarrassed to admit it, because she thinks it makes her vulnerable because Castle might figure out she's got feelings for him when he's dating someone else, and because she feels that she drove him away by not being honest with herself and with him about how she felt. I don't think she set out to hurt him at all. She just connected with this guy and got swept off her feet. Just like I don't think Castle meant to hurt her with Gina. He was just lonely and sad and jumped in with both feet. But do they both know they're in part to blame for the sticky situation? Absolutely.