The Unforgiven Curses are meant to be the worst thing a witch or wizard in the Harry Potter universe can do. Yet some students pulled them off.
The Unforgivable Curses in Harry Potter are exactly what they say on the tin: unforgivable. The Killing Curse, Cruciatus Curse, and Imperius Curse aren't worth using if you care about life, because the use of any can be enough to land you a spell in Azkaban prison for the duration of your time on the planet.
However, despite that, some students at Hogwarts opted to cast them. Sometimes for good reasons, other times for bad. So these are the times the Unforgivable Curses were used by Hogwarts students. Ironically, not a single one ever faced punishment for their crimes.
In the Half-Blood Prince, Draco Malfoy is given the task of murdering Albus Dumbledore by Lord Voldemort. The Dark Lord chooses to give this mission to the Slytherin student as punishment for Lucius Malfoy, whose failure to procure the prophecy earned him a place in Azkaban prison.
Draco hasn't got it in him to murder Dumbledore outright and uses the Imperius Curse on the bar lady of The Three Broomsticks, Madam Rosmerta. She's then forced to give a necklace to Katie Bell, who is then in turn meant to give it to the Hogwarts headmaster. Things go wrong, however...
While under the Imperius Curse, Madam Rosmerta passed on the necklace to Katie. She did so in the girl's toilets of the Three Broomsticks - and it was certainly necessary for Draco to do this, considering he had no chance of entering the premises.
Ironically, Draco was actually nowhere near Katie or Madam Rosmerta at the time of the incident. He cursed the latter a little while ago and had the perfect alibi, spending the day trip to Hogsmeade stuck working with Professor McGonagall. This is enough to keep the teachers off his back but Albus Dumbledore, in secret, knew what was unfolding all along.
In the Half-Blood Prince movie, Harry and Draco have an epic showdown in a girl's bathroom shortly after Katie's return from her traumatic injury. During this fight, Draco attempts to use the Cruciatus Curse - only for it to miss its mark.
Harry actually comes off looking like the bad guy after using Sectumsempra, a spell he'd read about in the Half-Blood Prince's own potion book. This seems to go unpunished in the film but in the source material, Severus Snape bans him from playing Quidditch. This means he misses Gryffindor's triumph, with star seeker Ginny Weasley in his place.
Harry, for the most part, sticks to his own code. He doesn't ever want to kill or injure anybody but, given how villainous the Death Eaters are, even he can't resist sinking to their level. And, after Bellatrix Lestrange heartbreakingly kills Sirius Black in the Order of the Phoenix, he chases the Death Eater eager for revenge.
During this pursuit, he casts the Cruciatus Curse. However, his attempt is feeble and Bellatrix doesn't feel any pain or fear. She soon changes, though, showing true terror as Lord Voldemort joins the battle at the Department of Mysteries, inside the Ministry of Magic. Bellatrix escapes with her life but feels the Dark Lord's wrath after failing to procure the prophecy her master desired.
The movie shows Harry attempting to disarm Snape but things are different in the book. The Boy Who Lived instead tries the Cruciatus Curse, only for his enemy to block it effortlessly. Snape ends up sending Harry to the floor and mocks him, revealing his true identity as the Half-Blood Prince.
During the events of the Deathly Hallows book, Harry is left with no choice but to break into Gringotts Bank with Ron Weasley and Hermione Granger in order to take Helga Hufflepuff's famous cup from Bellatrix Lestrange's vault. Yet though he and his friends take Polyjuice potion to become disguised, they inevitably run into trouble.
Death Eater Travers is one person who attempts to stop them but Harry, due to some quick-thinking, uses the Imperius Curse on him to force him into hiding. Things are different in the Deathly Hallows: Part 2 movie, however, with Harry using the spell on somebody else instead...
There's no Travers at Gringotts in the Deathly Hallows: Part 2 movie. Instead, Harry is forced to use the Imperius Curse on Bogrod just before the gang embark down to the vaults of the wizarding bank.
Harry does this while he's invisible and with Griphook the Goblin whispering in his ear. Griphook ends up betraying Harry, Ron and Hermione, attempting to flee and leaving them to the mercy of the Death Eaters. However, he lives to regret this when Voldemort catches up to him. The Dark Lord ruthlessly cuts down all who failed him and Griphook's body is visible in the aftermath of his murderous rampage.
While on the subject of Bogrod, it's only fitting to talk about Ron Weasley's own use of the Imperius Curse. When he, Harry, and Hermione pass under an enchanted waterfall the effects of the Polyjuice wear off, and the trio is exposed for who they truly are.
Bogrod notices this almost immediately, with the enchantment also undoing the Imperius Curse Harry had placed on him. Ron thinks quickly and is able to put him back under a spell. But the goblin ends up being killed by the dragon guarding the vaults - meaning Ron has blood on his hands.
Back to Harry, and the Boy Who Lived attempts the Cruciatus Curse again in the Deathly Hallows book. He does this on the repulsive Amycus Carrow, whose decision to spit in the face of Professor McGonagall inside Ravenclaw Tower sends the Chosen One into a frenzy of anger and rage.
This time, the curse succeeds in making its victim feel excruciating pain. The passage from the book reads: "The Death Eater was lifted off his feet. He writhed through the air like a drowning man, thrashing and howling in pain, and then, with a crunch and a shattering of glass, he smashed into the front of a bookcase and crumpled, insensible, to the floor."
None of the good guys dare use the Killing Curse throughout the events of the movies and books, no matter how high the stakes and how difficult things are. But Ron Weasley is the exception, giving it a go in the Deathly Hallows: Part 2 movie when himself and Hermione look destined to die via Nagini.
The curse just seems to go through Nagini like it's nothing whatsoever. Yet Neville Longbottom then has his golden moment, using the sword of Godric Gryffindor to decapitate Lord Voldemort's pet and destroy the Dark Lord's Horcrux in the process. This is key for Harry, who is then able to overcome his enemy once and for all.
