This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / The Wolf Man (1941)
Really, the last twenty minutes of the movie. Gwen accepts Lawrence despite all the, you know, murder charges when he sees the pentagram in her hand (meaning he's going to kill her Because Destiny Says So), and runs off, trying to protect her. This fails pretty miserably, and the only thing that keeps him from killing his one true love is being (slowly) beaten to death by his father with the same cane that had been a big part of his first meeting with said one true love. The sequels reveal that the experience was so heartbreaking that John died of a broken heart pretty much immediately afterwards.
And it's only made worse by the knowledge that this is just the start of what seems to be decades of suffering and torment as a werewolf. Those happy moments Larry has in the first part of the movie? They're the last times he ever smiles or laughs in his life.
For that matter, there's how Larry becomes the werewolf. He tries to save a young woman he barely even knows (and doesn't even seem all that fond of, given that he was hoping to be alone with Gwen Conliffe) from being slaughtered by a wild beast. And he not only fails to save her, but he gets suspected of murdering poor Bela AND becomes a werewolf!
Larry's reaction to seeing Bela's coffin in the church also counts. He watches from behind a pillar as Maleva speaks a blessing over her son's body. Then after she leaves, Larry comes out from his hiding place and collapses on the coffin weeping, clearly wracked with guilt for his role in the man's death.