During the "Final Scene", just after throwing Mrs. Lovett into the oven, Sweeney returns to his dead wife's side. In the original version of the scene, he sings a reprise of "Not While I'm Around", which has since been replaced with a reprise of "The Barber and His Wife". Either one fits this trope.
Sweeney Todd's reaction in general to him accidentally killing his wife. Imagine if you were in his position. You killed the person you love the most and you had no idea it was them the whole time. Some versions have him look so anguished as he stands over her corpse. Some also have him cradle her body before Toby slashes his throat. The concert version had him unbutton his collar, implicitly begging for a Suicide by Cop (well, by Toby).
The Johanna Quartet. While Anthony sings about wanting Johanna and Johanna sings about how excited she is to be eloping with him on Sunday, Sweeney Todd sings about letting go of his daughter and one of the last few threads of his humanity.
The "Johanna" reprise, which implies that as much as Sweeney misses his daughter and hopes to have her back, he really can't help but to feel a deep disconnection.
"Not While I'm Around" is more of a Fridge Horror tearjerker, since it all goes waaaaaay downhill from there.
Mrs. Lovett, trying to conceal her anguish as she tries to remain cheerful around Toby, sensing that bad things will come for Toby because he voiced his suspicions about Todd.
Anthony's and Johanna's final interaction on-screen. He assures her that they can leave all the "ghosts" behind. However, Johanna disagrees, believing that the ghosts would never die. Although it's implied they did manage to escape Turpin's clutches and move on, Johanna knows that the trauma will always haunt her.
The final scenes with Todd, taking the dead Lucy in his arms, and Toby slashing his throat, he's cradling her while slowly dying. Todd may have ended up becoming a psychopath, but the look in his eyes is the look of a completely broken man. Can you say Stoic Woobie?
Todd's reaction when he realizes the dead beggar woman is his wife. Usually in the stage productions, the actor playing Todd will scream and break down. But with Depp, his reaction is more quiet but with a growing sense of horror. He's finally gotten his revenge but at a terrible cost.