I watched the episode in question last night and I think from the way it was framed to the reactions of the other LEOs and FBI that the Prentiss scene was meant to disturb the viewer. She was freaking them out too. Though no one mentioned it, the case had to stress her considerably: she is worried Hotch won't come back and not as comfortable working with Morgan as boss, she's exhausted - they went from the funeral to the case without rest - and she fit the victim profile: mid-thirties to forties, attractive, well-educated, successful professional, single brunette woman from a high society milieu. She was identifying with the vics like crazy and when they went in that door and found that guy over the latest victim, knife in hand, the anger got the best of her. So she got a vicious satisfaction from taunting him after she put him down, not nice, but she is human and I don't think those words were coming from the FBI agent so much as the woman who imagined being at his (nonexistent) mercy; at least she didn't shoot him and I suspect she was tempted. She's still not (Future Vigilante Serial Killer) Elle Greenway.

I thought Paget Brewster really communicated the intensity and near meltdown very well - the camera guy got her eyes in close up beautifully.

I'm thinking this is the lead in for more back story for Prentiss. It's her turn to get hammered - because everyone in CM gets it sooner or later, even Garcia and she doesn't even work in the field!