Dean's always been pretty surly at 'monsters' and I didn't view him as being a dick, exactly? I thought he might be feeling guilt because of why Sully had shown up in Sam's life, and that was contributing to his dislike of the zanna, which felt like classic Dean to me. I'd have felt guilty too! Even though the poor kid was barely a teen, for Pete's sake; wasn't his fault.

All that being said, I do think this particular writer is a little ham-fisted when it comes to emotional arcs. Dean was a tad more crotchety than usual and Sam, a tad more maudlin. Neither of which bugged me much, but I did notice.