Well, this won't be a "happy recap" (as the late great Bob Murphy would say after the Mets won a game) but all in all, Disciple was a really good episode.
There were a few lighthearted moments:
the breakfast scene between Rick & Kate & the discussion of the honeymoon:
"Castle, are you using our honeymoon as an excuse to go and tour Middle Earth?" (after he mentions New Zealand)
and Rick to Kate:
"I love when you get angry. I mean at other people, besides me."
(which is kind of an echo of something he said to her in Season 4, Once Upon A Crime - "You're cute when you're angry. But not when you're angry at me.")
and then right before he brings up 3XK:
Beckett: "Since when do you ask for permission?"
Castle: "Touche."
and when they first talk to the plastic surgeon:
Dr. Nieman: "Can I just say you have an amazing face, detective?"
Castle: "And what about my face?"
Dr. Nieman: "Uh. amazing potential."
Carl: "You broke three of my ribs."
Castle: "I'm sorry, I was going for double digits."
and then a few more serious moments:
Lanie: "What if this is just the beginning?"
Espo: "It's going to be okay, I promise. I'm not going to let anything happen to you.
Lanie: "What if something happens to you?
Espo, to Ryan after Ryan is trying to get Espo to say how he feels:
"All right, fine. I want to find this piece of garbage, string him up and see how he likes it. There, I shared a feeling. Happy now?"
Beckett: "Nobody could survive that fall."
Castle: "Jerry Tyson could."
Beckett: "When Tyson escaped, he outsmarted all of us, but you've always blamed yourself."
Castle: "Because I'm to blame!"
I haven't had a chance to rewatch so I can't confirm if the homeless guy shows up again in the episode.
I have no problem believing that Carl did both murders, even if he did overdo it at the beginning.
I also have a feeling that Kelly & Tyson have been working together for a while. There's a part of me that wonders if she's the alpha in their relationship. Doesn't happen often, but female serial killers do exist. She gave off a creepy vibe from the beginning - I really thought she was the killer.
My thoughts on the song/ending/the look on Kate's face? Well, I keep changing my mind. First I thought the song had something to do with her mom. Then I thought it was nothing more than a creepy message to them. But then I thought it might be a message to her. Maybe when she was recovering after being shot (remember she spent months in her dad's cabin, cut off from everyone) she may have listened to that song or even that Dr. Strangelove is one of her favorite movies and she watched it while staying there. But whatever it was, she may have been alone when she did it and has never shared that with anyone. So it's a bit scary that he/she/they used it. And yeah, Kate's a badass detective who's not afraid of anyone, but things are different now. She's engaged, she's becoming part of a family, she may be thinking of having kids, and the thought of Tyson coming after her and killing her isn't so easily brushed aside. Plus it's possible that her PTSD kicked in. After all, Tyson framed Castle and she almost lost him last year.
No matter what, I have a feeling we won't know for a while what it meant. But that was a really good episode.
And I am really looking forward to next week's episode. It's the last new one of 2013.