The intent of the caster is a vital component of the spell. While Snape was still able to produce Avada Kedavra without any intent to kill, being a great wizard, the mix of hesitation and necessity probably resulted in the spell being blue.
The light of the Avada Kedavra curse is green. Some associate Slytherin with evil (this is not true) and their House colors are green. These are only a few reasons.
Green is one of the two most prominent spell colors in the story for sure, with the other one being red. This is based on the fact that the main character, Harry, uses the red Expelliarmus, while green is the signature color of his nemesis Lord Voldemort, when using Avadakedavra.
The Killing Curse, as an "unblockable" Curse, cannot be intercepted by another spell, except in circumstances of Priori Incantatem, where the caster and his opponent's wands and spells are locked together. An explosion or green fire may result if the spell hits something other than a living target.
The Headmaster's brilliance is practically limitless; more importantly, there are brands of magic that he would never resort to, regardless of the situation. He could have theoretically defeated Voldemort with a simple Avada Kedavra, but he chooses to err on the side of compassion, instead.
In order to conjure the avada kedavra curse, you have to want to kill your victim. We all know that Voldemort could easily kill a child without an ounce of remorse... but not Snape. Snape didn't want to kill Dumbledore, and this was why the spell was blue instead of the usual green.
Simple answer is, that's only in the movies. They often play fast and loose with things that might look or sound cool in the moment but make no sense in the overarching story. Ron never casts Avada Kedavra at Nagini in the books.
Family. Snape's family background is mostly shown in flashbacks during the course of the last three novels. Snape was born to Eileen Prince, a witch, and Tobias Snape, a Muggle, making him a half-blood (hence the name, "Half-Blood Prince").
The Disarming Charm became Harry Potter's signature spell, and it notably caused the final defeat of Lord Voldemort during the Battle of Hogwarts by rebounding his Killing Curse, because the Elder Wand recognised Harry as its master through the spell connection, and thus strengthened the Disarming Charm to the point ...
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.
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.
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.
What color is expelliarmus? In the first three movies when Harry uses expelliarmus it looks blue but in goblet of fire when he and voldimort battle it's red.
It's also believed that Avada Kedavra requires the caster to be willing to commit murder for the spell to work. While it was difficult to master the Unforgivable Curses, Harry might not have had the ability or desire to use the Killing Curse since he's not a murderer.
Yes, a skilled witch or wizard can counter or block the Expelliarmus spell, using spells such as **Protego** (the Shield Charm) or by simply evading it. The effectiveness of the defense often relies on the caster's skill and timing.
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.
The author explicitly states that both women were fighting to kill, despite Molly refraining from casting the Unforgivable curse Avada Kedavra. The result of the duel actually comes about as a result of nothing more than spectacular accuracy on Weasley's part.
In Harry Potter and the Deathly Hallows: Part 2, Voldemort may have used this spell to slit Snape's throat before having Nagini finish him off. The effect of Voldemort's use of the spell was a slash of the Elder Wand and it cut Snape's throat.
Snape having the same Patronus, the doe, as Lily symbolized his everlasting love and unwavering devotion towards her. He hadn't stopped loving her after their last altercation, marriage to James Potter, or death. She was his "always," his guiding light, and he protected Harry for her.
