While another poster has made valid points about holes in the plot, when he or she states that "I believe that Cinco de Mayo is not widely celebrated in Mexico and it was therefore wrong of the producers to assume that this would be a normal thing to do", I had to wonder if that means the United States doesn't celebrate the 4th of July. Absurd comment.
Yes, Elle's Spanish is lacking. The character could certainly be of Cuban descent on her mother's side (we are not talking about Suzanne Somers here) and the use of "tu" instead of "usted" would be an example of a cultural difference. As far as speakers of foreign languages miraculously speaking in English (sometimes perfectly), whether they are supposed to be Spanish or Arabic or Chinese, that happens quite often, most likely out of expediency as translator characters are not always built into every scene of every episode of procedural programs (NCIS would be a perfect example, as well as JAG, Person of Interest, as well as Criminal Minds).
Now, to get to the meaty part of this review. I found it compelling and, as far as I know, reasonably realistic, again aside from some of the flawless English of certain characters. The episode, one scene of which, I am sure anyone who has seen it will know which, borrows a twist from Alfred Hitchcock's Psycho, is too politically incorrect to have been made today. The use of the word "homosexual", for example, which has fallen into almost complete disuse due to objections from the LGBTQ community.
The final scene, while satisfying, contains both a clear goof and an amazingly blatant reference. I will explain. The "goof", as far as I am concerned, is how the surviving female victims of a serial rapist and killer somehow knew before the BAU where the next killing was set to take place. That is never explained. The amazing part, which might or might not be possible in today's media world, is the fairly blatant reference to castration, illustrated by a knife being tossed down in front of the arriving police and BAU by the leader of the victims turned avengers.