Harry became so enamoured with the spell that he eventually used it to finish off Lord Voldemort. Here's how Harry's relationship with Expelliarmus became his signature – and why that's a good and bad thing.
In a duel, it is a logical and effective way to end the fight without going to the extremity of killing your opponent. Harry's intention was to STOP Voldemort, if it could be done without killing him than that was to be preferred.
So why does Harry always use Expelliarmus? Well that would be just his natural reaction. From the wiki page harrypotter.wikia.com/wiki/Harry_Potter it says that Expelliarmus is his signature spell and would thus be his natural response to any attacks. Just like if person A trained the hammer fist a lot.
Harry's spell simply rebounded a weak killing curse back onto Voldemort who was extremely weakened with all of his Horcrux's destroyed. The Elder wand couldn't conjure a full power curse for Voldemort. Especially against its master.
During his final confrontation with Voldemort, Harry knew that he (Harry) was the Elder Wand's true master. He again cast Expelliarmus, ejecting the wand from Voldemort's hand into his own as the Killing Curse rebounded off his body, fatally striking Voldemort.
Throughout the entirety of the franchise, Harry is thrown into the most heat, and his skills as a wizard are constantly tested. That being said, his go-to spell has to be Expelliarmus.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
Lily's Protection In Voldemort's Blood Saved Harry
When Lord Voldemort returned Harry Potter and the Goblet of Fire, he used Harry's blood - which is partially how Harry survived Avada Kedavra in the forest.
The darkest of the Dark Arts, Harry Potter unforgivable curses are the Cruciatus Curse, the Imperius Curse, and the Killing Curse. Although the curses are highly illegal, Harry using Unforgivable Curses happened, but he uses them once he was thrust into the Second Wizarding War.
It was Snape who taught Harry his signature spell: Expelliarmus.
So, Voldemort had to kill Snape for the wand's allegiance to switch. Voldemort knew he couldn't cast Avada Kedavra with the Elder Wand to kill its master, so he ordered Nagini to be the one who killed Snape. In reality, the Elder Wand belonged to Draco, as he was the one to disarm Dumbledore in the Astronomy Tower.
The Killing Curse is a spell that causes instantaneous death and is one of the three Unforgivable Curses. Its incantation is Avada Kedavra. The only known counter-spell is sacrificial protection, which uses the magic of love.
He is also revealed to be a double agent for Albus Dumbledore and the Order of the Phoenix. But many people forget that he also invented a number of spells. In the Harry Potter movies, Snape came up with the Sectumsempra curse when he was a student at Hogwarts, and it's known to cause painful lacerations on the target.
Draco Malfoy's mother Narcissa was cold, cunning and devoted to the Dark Lord. But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
Speak to Sebastian in the Undercroft after this and he'll teach you any Unforgivable Curses you have missed up to that point, including Avada Kedavra. It's worth learning the curse as doing so should have no negative outcome on the game's story.
Is Hagrid immune to Avada Kedavra? Hagrid is resistant to spells, but not immune. AK is explicitly said as having no countermeasures known, and while that isn't strictly true, half-giants are common enough that, if they were immune, it would be noted.
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.
Voldemort killed Snape in Harry Potter And The Deathly Hallows Part 2 using his Horcrux snake Nagini instead of his favorite Unforgivable Curse, Avada Kedavra.
Avada Kedavra
Topping the list for the most powerful spells in the Harry Potter universe is no other than Avada Kedavra, Voldemort's favorite to instill his will and fear onto others. The spell itself is a play on Abra Kadabra and is easy to cast by just about anyone in the Wizarding World.
Expelliarmus. It was this spell, this spell that won Draco Malfoy his 1st duel against Hermione Granger.
Cruciatus Curse ('Crucio')
Harry tried to use this spell himself. The first time was against Bellatrix after she had killed Sirius. His attempt was unsuccessful because, as Bellatrix mockingly explained: 'You need to really want to cause pain – to enjoy it – righteous anger won't hurt me for long…'
As the youngest Weasley, we're sure Ginny observed more than her fair share of unpleasant spells over the years but it was the Bat Bogey Hex she was famous for – and with good reason.