Break the Cutie/Theater

Tosca: Floria Tosca is a sweet, religious girl, though a bit prone to jealousy. Corrupt police boss Scarpia uses this jealousy to not only get her to accidentally betray the artist Mario Cavaradossi, who she loves, to him, but then forces her to both tell him where Mario might be hiding Angelotti to stop him from being tortured, then agree to be raped to keep him from being executed in Scarpia's namesake ultimatum. Poor Tosca has a complete breakdown at that point, asking God why he would do this to her, who lived only for art and love, and tried only to serve him. She manages to palm a dagger and kill Scarpia when he returns to rape her -- but, when she goes to meet up with Mario, the false execution that Scarpia arranged... turns out to be not so fake after all. As she breaks completely, and the troops can be heard coming to arrest her for the murder of Scarpia, she takes the only action left to her, and throws herself off the roof of a tower.
- Opera in general is a veritable breeding ground for breaking cuties. And Puccini was a GREAT cutie breaker. Poor, poor Cio-Cio-San from Madame Butterfly, indeed. And if we go to Donizetti, Lucia di Lamermoor. And, in French opera, Marguerite in Gounod's Faust.
  - As that distinguished opera critic, Bugs Bunny said in What's Opera, Doc?, "What did you expect from an opera? A happy ending?"
  - Puccini certainly put his cuties through the wringer. Poor Sister Angelica, forced to live out her days in a convent for the crime of having a child out of wedlock seven years prior: one day her rich aunt comes and tells her, "Your younger sister is about to get married to THE MAN YOU SLEPT WITH, you brazen slut, and you have to sign over your inheritance to her, since you won't be needing it. Oh, and your son died a couple of years ago. Bye," after which Angelica, devastated, brews up a poison out of the plants in the garden she tends, drinks it, and then realizes that she's committed a mortal sin and therefore has condemned herself to hell.
Maria in West Side Story.

"How do you fire this gun, Chino? Just by pulling this little trigger? How many bullets are left, Chino? Enough for you? And you? All of you? WE ALL KILLED HIM; and my brother and Riff. I, too. I CAN KILL NOW BECAUSE I HATE NOW! How many can I kill, Chino? How many -- and still have one bullet left for me?"

- Anita might also count as this. She starts out as a nice girl who's happy to be in America and only wants to live her life in a new country and even help Maria and Tony out. Then Tony kills her boyfriend. Understandably she becomes pretty jaded, but she STILL agrees to help Maria get a message to Tony (yes, the guy who killed her boyfriend). Instead she finds his friends, who almost rape her. That's pretty much the last straw, and she tells the lie that leads to the tragic conclusion.
Shelley in the musical Bat Boy. She is living happily with her veterinarian father and house-wife mother. She falls in love with Edgar, a feral 'Bat Boy' that was found nearby who lived on blood, after her mother makes him civil and caring. All is well. Towards the end of the show, after Shelley and Edgar have already had sex, it's then revealed that while her mother worked as her father's assistant in a lab, he accidentally spilled an experimental pheromone on her, and, driven mad with lust, raped her. Stumbling her way home, she is then attacked and raped by a swarm of bats.She got pregnant and gave birth to twins: a mutant (Edgar), and a human (Shelley) So, Edgar turns out to be her twin brother. In the finale, Shelley watches as her father (Dr. Parker), furious and out of his mind, slits open his throat to tempt Edgar with blood. Edgar pounces on him and begins to drink, and Dr. Parker takes the initiative to stab him multiple times in the back. Trying to get him to stop, her mother rushes in and is also stabbed by her husband. All three of them die, and Edgar dies in her lap. So, no only does she find out her lover is actually her brother, she watcher her ENTIRE FAMILY get murdered by her father, who in turn bleeds out through the neck. After that, she's very quiet. Nice story, right? I thought so too.
Hamlet pretty much pulls this trope on Ophelia. Between his running into her room disheveled, sexually harasses her (in two separate scenes, no less) and finally kills her father under the impression that he was killing Claudius, driving her insane. It's not quite Kill the Cutie, since she kills herself, but it's at the hard dark edge of the two tropes.
While Elphaba from Wicked isn't exactly a traditional cutie, the Wizard and Madame Morrible do their best to break her by turning her into a terrorist fugitive, murdering her boyfriend, giving her former best friend an important position which seems to be entirely trying to mitigate Elphaba's activities and DROPS a house on her sister. No wonder she finally snaps in epic fashion during 'No Good Deed.'
Philomele in The Love of the Nightingale starts out as a beautiful if naive girl. Her sister and best friend Procne asks her husband Tereus to bring her for a visit. Tereus proceeds to fall for Philomele who in turn has fallen for a captain. Tereus then tells her Procne's dead, kills her lover, proceeds to rape her after she turns him down and finally cuts out her tongue to keep her from talking. She spends five years alone but with her servant before finally making dolls to reenact what happened to her to Procne. Tereus tries to kill them but they (as it is a Greek myth) turn into birds, making them a literal Broken Bird
In Vanities, Kathy's boyfriend Gary, while she was on the pill, slept with another girl, got that one pregnant, and married her. Kathy is utterly and irrevocably devastated as a result, to the point of having a nervous breakdown. Cute Boys with Short Haircuts says it all.
By the end of Sweeney Todd: The Demon Barber of Fleet Street, Johanna and Toby have gone mad due to having been treated cruelly for their whole lives, but also partly because of their first hand discoveries of Sweeney and Lovett's practices.
Juliet in Romeo and Juliet. Three hours after she marries Romeo, he kills her cousin (who was like a brother to her) and gets kicked out of Verona. The next day her parents try to force her to marry a man she doesn't love, and threaten to disown her if she refuses. Her father claims that he and her mother "have had a curse in having her". Meanwhile, she's loyal to Romeo, not only because of her inclinations, but because of her religious beliefs (i.e. "I'm already married, it would be wrong of me to get married again") and is fully prepared to kill herself rather than go through with the wedding. She ends up taking a potion that makes her appear dead, even though she's terrified of what it will do to her, as part of an incredibly risky plan to get out of Verona that entails never seeing the people she loves again. Then, when she wakes up in her family tomb, her husband is lying dead with his head on her chest. She runs herself through with his dagger. And she's fourteen years old.
Aww, look at Seymour with his Adorkable clumsiness, his hopeless little crush on the local abused girl, Audrey, and his quirky way of confiding in his pet plant! The poor guy's lived such a horrible life, and no one's ever loved him--can you really blame him for wanting a little happiness? Or for wanting to off the jerk who beats up Audrey? Or for having to kill off his boss, too, so he won't get ratted on, and then keeping the plant who needs to be fed blood so that Audrey won't leave him and ...well, you see where this is going. Suffice it to say that his death is probably the nicest thing that happens to him in the whole show.
Kim in Miss Saigon. Orphaned when her family's village is bombed. Has to go to work as a prostitute to support herself. On her very first night of work, meets and falls in love with a disillusioned GI (who himself may be an example of this trope). He plans to take her home with him, but instead, they are separated in the chaos of the fall of Saigon. She has to endure pregnancy and childbirth on her own. She has to kill her cousin to protect her child from him. After three years of pining away for Chris with her faith in him and love for their child being the only things that kept her going, she rushes to his hotel room. . .to be greeted by his wife. Determined to make sure that they take the boy with them to America (as a half-Asian, he would be an outcast in Vietnam), she kills herself. Yeesh.
Next to Normal is pretty much Break The Cutie: The Musical. Diana has been treated for severe Bipolar disorder for twenty years, haunted by manic-depressive episodes, side-effects of potentially lethal medications, and hallucinations of her late son. After undergoing a heavy dose of ECT (which she seems to have enjoyed a bit too much), she loses most memories of her family. After regaining them, the trauma of losing her son finally rushes back she decides that the slow and dangerous treatment isn't worth tearing her family apart, so she decides to leave them.
In A Streetcar Named Desire when Stanley raped Blanche the cutie really became broken.
Spring Awakening is also a good candidate for Break The Cutie: The Musical because half the characters get broken. Ilse and Martha got physically/sexually abused by their fathers, and they both had crushes on Moritz. Speaking of Ilse, she got kicked out for telling someone about her father and ran off to an artist's colony, only to head back home when one of them holds a gun to her chest. After heading back and conveniently meeting Moritz, who also got kicked out for failing in school, she offers to take him home--but he refuses. After realizing he's made a huge mistake, he eats his gun. Meanwhile, Wendla gets pregnant by Melchior and later gets a botched abortion, which Melchior is unaware of because he got framed by the teachers and sent to a reformatory for his best friend's suicide. Then there's Hanschen and Ernst, who... kind of vanish after the second act.