Harry didn't use that spell because he cannot bring himself to kill anyone. He is not a killer. Just like in the muggle world, it is very difficult to bring one's self to kill another human being.
Harry Potter Killing Bellatrix Wouldn't Have Helped Voldemort. Unlike the dark side of the Force in the Star Wars franchise, the Wizarding World's Dark Magic does not have addictive properties, so one use of the killing curse on a ruthless murderer would not suddenly turn Harry Potter into a dark wizard.
When Death Eater Bellatrix Lestrange killed Harry's godfather Sirius Black, Harry chased down Bellatrix and performed the curse on her out of rage. However, it was performed unsuccessfully as Harry was doing it with "righteous anger" and couldn't muster the will to actually cause her harm.
During the Battle of the Department of Mysteries, Harry tried to use the Cruciatus Curse on Bellatrix Lestrange as a means of vengeance for killing Sirius Black, but it was ineffective, because he was doing so out of "righteous anger".
Why Harry DIDN'T Use Avada Kedavra on Voldemort - Harry Potter Explained
Why is Imperio unforgivable?
After the Wizards' Council was reformed into the Ministry of Magic, tighter restrictions were placed on the use of certain kinds of magic. The Imperius Curse was deemed by the Ministry to be Dark magic, and, along with the Cruciatus and Killing Curses, were declared "unforgivable" in 1717.
In the film, Bellatrix casts a blue spell at Ginny Weasley instead of a Killing Curse, who deflects it using a Shield Charm. In the book she just avoids it.
Effect. Unforgivable Curses are the three most powerful and sinister spells known to the wizarding world, and are tools of the Dark Arts. They were first classified as "Unforgivable" in 1717. They are the Killing Curse, Avada Kedavra, the Cruciatus Curse, Crucio, and the Imperius Curse, Imperio.
Although it is not confirmed to have been the Killing Curse, it is highly likely that Ron attempted to kill Voldemort's snake, and last Horcrux, Nagini, with Avada Kedavra. This is because, in the movies, Ron can be seen firing out a green spell at the snake, and it looks just like the Killing Curse.
During the Battle of Hogwarts, Voldemort used the Cruciatus Curse on what he believed to be Harry Potter's dead body as a way of dishonouring it and to show he had won. Harry felt no pain because the wand Voldemort used, the Elder Wand, refused to harm its true master.
Near the end of Harry Potter and the Deathly Hallows, Bellatrix Lestrange shot a curse that nearly hit Ginny Weasley, and this inspired Molly Weasley to step up and prepare for a duel to the death.
Originally Answered: Did Bellatrix feel sorry for killing Sirius? Bellatrix isn't sad about killing Sirius. She taunts Harry about it. Far from showing remorse for killing Sirius, she next stalks her niece, Nymphadora Tonks.
Though Narcissa may not necessarily have agreed with all of bellatrix's life decisions, and her views, she certainly loved her sister, and will have certainly mourned her death.
The Killing Curse was created in the early Middle Ages by Dark witches and wizards. Primarily, the curse was used to quickly slay opponents during a duel. In 1707, the Wizards' Council was restructured into the Ministry of Magic, which allowed for more strict restrictions on certain types of magic.
However, despite being rescued by them, Griphook's general mistrust of wizardkind resulted in him betraying Harry Potter, Ron Weasley, and Hermione Granger. He betrayed them because he believed they would not hand him the Sword of Gryffindor, after destroying the Cup and other Horcruxes.
There is likely a good reason that the Sectumsempra is not an unforgivable curse. It's mostly because Sectumsempra is Snape's personal curse that he created. It is not a widely known or used spell, and it is one that he kept secret — which means it can't be added to the list since no one really knows about it.
It did backfire on Voldemort but just not the degree it did the first time. It failed to destroy his body because it killed the piece of hit soul that was in Harry forming the horcrux but it did knock him off his feet and actually rendered him senseless.
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.
Harry used the Imperius Curse multiple times during the Gringotts heist to accomplish their mission. Harry never used the Killing Curse, Avada Kedavra, due to his moral stance against murder and his personal connection to its victims.
In the final movie, Harry Potter and the Deathly Hallows pt. 2, Molly first hits Bella with a spell that freezes her, most likely Petrificus Totalus. Then, she launches another one that shatters her entire body into pieces, which could be expelliarmus, bombarda, stupefy, or any spell that can produce such an effect.
Molly Weasley, although not the most powerful witch, was a member of the Order of the Phoenix, indicating her strength and abilities. Molly's ability to cast non-verbal spells demonstrated her mastery of magic.
Voldemort stored the his 16 year old memory in the diary in the form of horcrux. Ginny got this diary in custody through Lucius Malfoy and from then on Riddle started controlling her actions. Voldemort has the power to look into others' mind and control them like he did with Harry in Order of the Phoenix.
Delphini was born in secret at Malfoy Manor in early March 1998 as the result of a liaison between Bellatrix Lestrange and Lord Voldemort. On 2 May 1998, both of her biological parents were killed in the Battle of Hogwarts.
