Terry's death. Bad enough that he's yanked through the window by one of the werewolves, but the poor guy doesn't actually die until Joe gets to the barn and sees the wolf feeding on him, meaning he was suffering horribly until then. To make it worse, he weakly reaches for his friend to help him before his head gets ripped off and thrown at Joe.
Sarge's last moments, namely where he looks at the picture of his wife for the last time. Especially sad when you realize earlier in the movie he'd said the thought of never seeing her again is among the things that scare him the most.
And the scene earlier when Sarge quietly revealed to Cooper that he was becoming a werewolf and made it clear that he intended to go down fighting rather than succumb to the bloodlust.