I normally don't review episodes, but I don't know, something made me want to review this one. Anyway.
Note before I begin: I've skipped episodes (two now) based on things in them, so if you're like this too, please note that this episode has (spoilers here, and throughout the review!): suicide (cutting) seen on screen (teenager), hebephilia (the teenage version of pedophilia -- techinally not seen on screen, but really hard to ignore), abduction (of a grown woman and of a child), sexual assault, erotomania (delusion disorder centered around a falselyperceivedlove), restraints (seen, but not used in the episode, though it's clear they've been used before), hints of father/daughter incest (not happened, but implied was his eventual intent)and I think that's it. Oh, and abusive relationship? Er, between the unsub and his "wife" (not seen, but pretty obvious).
Going forward: this episode is Garcia-centric, and I think it sucessfully emphasized herparticulartalent of empathy and sheer amount of heart. Not that the other characters don't have these qualities, mind you, but Garcia has this in excess, and is very open for someone working in a unit that tracks down serial killers. It's this quality that allows for her to bring attention to Monica and her abduction fairly quickly, and ultimately save her life when talking to the killer.
The killer this time is unsympathetic, and I honestly don't think you're supposed to emphasize with him at all (which is why Garcia's empathy - or pretend empathy - at the end is all the more impressive). Sure, you could argue that his motive is love gone to the extreme, but once you kidnap a child, keep her captive, rape her and ultimately drive her to suicide in the name of love, you've pretty much destroyed any sympathy possible. (Others may find this different, and he is suffering from a delusional disorder beyond his control, but screw it, the episode following Monica like that makes it pretty much impossible for me to empathize with him at all).
This episode manages to show the ugly and the good parts of positive human emotions: the ugly in the unsub's obsessive "love", the good in the power of Garcia's empathy (mentioned above) and her dedication to helping others through grief, and the courage of a mother in an impossibly hopeless situation.
There was some fine little details that added to the horror of this episode: the obvious unhappiness in the pictures of the victim at the unsub's house (compared to the pictures seen in the mother's old house), the ring on the unsub's finger (that sent chills down my spine at the end of the episode, because it just reinforced the sickness of the whole thing), the scratched-in name at the table, the restraints, the "why me" on the wall -- lots of little details that made my stomach turn beyond the obvious sickness and horror of the episode.
The scene with the friend was a nice detail and added some depth to the entire story, and the case-wise, this was a fascinating episode - very unexpected. It worked for me, but I can't say it will work for anyone.
I personally found the butterfly metaphor a bit heavy-handed, but that might just be me. I'm better at judging writing than execution, but I thought it was overall well-done.
As for the father/daughter incest hint thing mentioned in another review: it didn't go over the line for me, and it made sense in context, since the original father was dead, the man was obviously disconnected from reality, and adding in another person would have complicated the whole episode even further. YMMV.
Finally, I would have liked to see them doing something (I have no clue what) with JJ and the butterflies, since it was mentioned she collected butterflies as a kid. I don't know, I just think the writers have missed some connections/emotional reactions from the characters when it comes to the backstory and plot (see also last episode with Reid and the whole "mentally sick mothers who devote themselves to their children" thing that was an obvious connection to his backstory and yet the backstory didn't even get a mention.) Anyway, I'm rambling now, so this review ends here.