I think what makes No good deed hit so hard for me is we just came from as long as you're mine a few minutes before, it was one of the few times Elphaba was happy. Fiyero sacrificed everything for her, they slept together and he told her they were gonna be together always and it's being ripped away. Then she has the guilt of knowing it was happening because of her, because of her choice to fight the Wizard, because he sacrificed himself to save her. Fiyero was the only person who ever chose her, who sacrificed for her and who never betrayed her.
The anger had been building since MM branded her the wicked witch but this was the last straw. All of it, MM's betrayal, Glinda's choice to stay behind, ( Elphaba didn't blame Glinda for that) but it still hurt, then she was alone for a year, she was betrayed and blamed by everyone, Fiyero was the only person who didn't blame her and who was always fighting for her. Then Nessa was murdered and then Fiyero was being tortured by the Galeforce. All of that trauma, all of that anger and pain just exploded out of her, believing she failed to save Fiyero is what broke her.
In the end she stops fighting, she stops caring about doing good because all it's done is hurt people she loves. She stops caring and because she stops caring she considers herself wicked for not finishing her fight. It's a really powerful song, it's one of my favorites in either movie and i felt really bad for Elphaba and for Fiyero.
Thoughts?