Dean and Sam figure out that a reaper was involved, but they originally figured that Roy was behind it all. It wasn't until they saved a protester of the sessions (while a woman named Layla Roarke was called up) that Sue Ann was revealed as the culprit, with Dean catching her with an ancient cross used to summon the reaper.
Sue Ann's evil game plan was to kill people that she deemed were "wicked," and after she catches Sam in her attic, which has photos of people she has killed and a mark next to Dean, she locks Sam in. She sends the reaper after Dean, while Layla is again attempting to be healed of a brain tumor, but Sam escapes, removes Sue Ann's cross and destroys it. After this, the reaper goes after Sue Ann and kills her.