This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Once Upon a Time in Wonderland
Episode 9 - Nothing To Fear
Trying to stop an angry mob from killing her, the deposed Red Queen suggests that they instead hold her for ransom. The leader of the mob asks who would pay to save her life. The Queen can only answer "No one."
The Knave rejects Lizard's love because he doesn't have a heart. As if that's not bad enough, Lizard says "I wish you could feel something for me" while still holding the bottle. She drops dead and the Knave is sucked back into the bottle because she used her last wish, feeling the only thing he can feel for her now: grief and anguish.
Episode 10 - Dirty Little Secrets
After sincerely trying to make up for all the bad things she'd done, Anastasia is broken by the Jabberwocky and makes her three wishes, allowing Jafar to become Will's master and begin the spell.
Cyrus' backstory. After Cyrus cheats two men in a card game, they set fire to his house in retribution. His mother is badly burned. Then he pressures his brothers into stealing magic water, which heals their mother. However, the guardian of the water curses all three of them into becoming genies.
Episode 11 - Heart of the Matter
The ending. Will gets his heart back, he and Anastasia share a beautiful kiss...and then Jafar fatally stabs Anastasia, who dies as Will cries and screams from behind bars.
Especially given the echo of the earlier scene when Jafar threatens Anastasia to get Will to talk - Will barely cares, and Jafar is drawing blood before he speaks up.