This episode is not as bad as people say it is, and I'm confused why people are so upset about it. I thought this was a great episode, and the tension between the two lovers was intense. I don't see how it was poorly-acted; every time they were onscreen, I was wondering what would happen next.
However, there are two big flaws here. It is a ripoff of Natural Born Killers, and the episode focused on the killers more than the BAU. The point of CM is to examine the BAU as well as the killer(s), and it shows that even the ones that want to help us have that dark urge in them. So, spending more time on the killers fucking each other rather than examine the BAU as well is frustrating.
However, it still has the unsettling atmosphere and intense chemistry between the killers and their victims. "The Thirteenth Step" misses a few steps, but it's still worth a watch.