The episode is about a killer who puts his victims on the rack to stretch them and crucifies them.
Transcript :
Dr. Spencer Reid : So far we believe he has spared the one woman that he still has in captivity. She's either witnessing these horrors or being forced to participate in them.
Dr. Alex Blake : Crucifixion is sadistic and watching it is the ultimate torture.
Whew, the guy who got crucified and murdered sure was lucky he didn't get tortured as badly as having to watch it happen to someone else.