From the beginning, I thought they were setting up Stephanie, the matronish young woman who was 'using this incident to get nosy' as the Real Killer, especially when JJ's questioning reveals that female students have been carrying stuff like mace, well, a taser is carried by some women as a self defense weapon, whether she's supposed to or not. The brunette girls have been openly disrespectful and mean to Stephaine, heck, I think one of those scenes came after the BAU said that rejection was a common theme in the unsub's life - Stephanie looked and acted like the girl who was rejected a lot, like Principal Snyder ish.
Nathan Tubbs was rejected from the army because he had borderline, antisocial and sociopathic tendencies, our glimpse of him posing with his guns in his room screams "anxious masculinity". I can see this antisocial sociopath pushing a woman around, scaring a girl he's on date with by ranting non-stop about his ex-wife, beating someone up 'because she provoked him', and feeling no remorse for it. I can't see him, Mr.MessyLockerBoozer, being organized enough to stun someone, kill her quickly with a knife to the heart, before moving onto stabbing the /corpse/.
I can see someone like Stephanie being housecleanerish, she's killing those mean irresponsible girls who are pregnant, before they can give birth, she doesn't want to torture them, hence the quick death, but she's angry, so much suppressed anger, hence the post-mortem stabbing. Then she feels bad because she's supposed to be looking out for them and she just killed someone pregnant, hence the posing.
Stephanie would know which girl was alone outside, after curfew, disobeying her good advice. ...and they might make fun of her, but after standing alone in the dark worrying about a slasher, I would smile too if I see mean and strict matronly woman checking up on me.