The devil put his finger upon us
Dean will never understand humans. There's already enough evil in the world happy to tear it apart, so he doesn't get why they've gotta kill and hurt each other, too.
He doesn't expect to find anything, breaking into the dead guy's house. Just some poor bastard killed by a crazy drunk.
So when Sam cracks open the email and they learn the poor bastard was actually a sick fuck, Dean's stomach twists.
He doesn't understand humans, and sometimes he almost forgets why they're worth saving.