The Alpha pack’s been roaming Beacon Hills and causing trouble for weeks, resulting in several deaths among the humans. When the Winchesters arrive to town, however, they manage to capture Derek; unaware there’s more than one pack in the territory.
And Dean - newly returned from Hell - doesn’t care which monster is guilty of what crime.
"Hey, Empire State Building! Derek didn’t kill those people!"
"How can you be so sure, kid?"
"Because he’s my mate. I if anyone would know."
"I thought you guys were supposed to be experts?"