Gatekeeper

A man is so traumatized by his own failure to protect his son that he starts killing people he think might someday harm his pregnant estranged wife and the baby. He photographs his victims, seemingly to keep them from seeing his face, leading the BAU to profile him as a voyeur. A woman, Ashley Fouladi, accidentally discovers the UnSub's room of victim photos, and in the subsequent struggle she falls to her death. When her body is found covered with a blanket, the BAU realize the UnSub isn't a voyeur so much as he is stalking everyone who lives in that one building. This narrows the suspect pool so much they pretty immediately find out who he is, and when they arrive at his place, that he is spying on his pregnant wife who lives with her brother.

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Tropes in this episode:

• Comically Missing the Point: JJ says she believes no one on the force has heard Rossi sing. Rossi says that was intentional.
• Crazy-Prepared: During the climax, Spencer has to help the UnSub's ex-wife deliver a baby. When asked how he knew what to do, Spencer explains that he memorized labour manuals in case J.J. ever went into labor in the field.
• Delivery Guy: Spencer Reid helps Hannah deliver a baby. He's a little more prepared than the average given it's, well, Reid.
• Dramatically Missing the Point: Tanner refuses to drive Hannah to the hospital when she goes into labor, too fearful that something could go wrong.
• Grief-Induced Split: The UnSub's wife kicked him out two months before his killing spree started, it being fueled by the grief of not having been able to protect his son.
• Imperiled in Pregnancy: Hannah is alone with the UnSub while giving birth.
• Ladykiller in Love: One of the subplots of the episode. One of the victims was a habitual womanizer, but he had started developing genuine feelings for Ashley and considered having a serious relationship with her. The killer attacked him before he had a chance to do so, because the killer was convinced that the guy would corrupt Ashley.
• Metaphorgotten: Garcia declares that she has earned a diamond with her excellent research. After she relays her information, Morgan says it's more like a cubic zirconia.
• Screaming Birth: Hannah goes into labor and gives birth at home in this style. Her screams are so loud that they're heard outside.
• Shout-Out: The team, minus Hotch and Blake, sings "The Piano Man" by Billy Joel.
• Stalker without a Crush: The UnSub stalks the people living in his apartment building and kills all those he considers a bad influence on them.

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Reid: "The most important thing that parents can teach their children is how to get along without them." Frank Clark.