Why was Harry casting shield charms against Voldemort in the Battle of Hogwarts?

Question

As we know from Defense Against Dark Arts, year 4 (as told by Barty Crouch Jr impersonating Alastor Moody), Avada Kedavra can not be blocked:

"Not nice," he said calmly. "Not pleasant. And there's no countercurse. There's no blocking it. (GoF, Ch 14 - THE UNFORGIVABLE CURSES)

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Yet, in the middle of a battle, when Voldemort was threatening the lives of people, Harry did in fact cast blocking curses.

Now, this is Harry Potter we are talking about - not necessarily the greatest academic mind, but a very gifted duelist with excellent on-the-spot thinking and usually correct reactions.

So why was he casting blocking spells that he knew full well would not work against Avada Kedavra, instead of, for example, attacking Voldemort ASAP to distract him?

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First, right after Neville killed Nagini and Voldemort was about to kill him:

Hidden beneath the Invisibility Cloak, Harry cast a Shield Charm between Neville and Voldemort before the latter could raise his wand. (DH, page 733)

Please note that this was fairly clear that the Shield was to protect against the killing curse; since Voldemort already tried to hurt and torture Neville, and killing him for destroying his las Horcurx would have been the only predictable response for Voldemort.

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Second, in a similar circumstance, to protect Molly once she killed Bellatrix:

Voldemort raised his wand and directed it at Molly Weasley.
“Protego!” roared Harry, and the Shield Charm expanded in the middle of the Hall, and Voldemort stared around for the source as Harry pulled off the Invisibility Cloak at last. (DH, page 737)

Same story - Voldemort was super-mad and definitely about to kill. So casting a blocking shield curse that would have blocked some other spells would have been a waste of time/effort.

Answer 1

There is one thing that can block Avada Kedavra and that's the power of love. Lily, through her sacrificing her life to protect Harry, caused the curse to rebound against Voldemort. And, later, Harry willingly sacrificing his own life to Voldemort in the Forbidden Forest caused a similar phenomenon to occur: those who Harry loved, cared about, could not be injured or killed by Voldemort's spells.

‘You won’t be killing anyone else tonight,’ said Harry as they circled, and stared into each other’s eyes, green into red. ‘You won’t be able to kill any of them, ever again. Don’t you get it? I was ready to die to stop you hurting these people –’

‘But you did not!’

‘– I meant to, and that’s what did it. I’ve done what my mother did. They’re protected from you. Haven’t you noticed how none of the spells you put on them are binding? You can’t torture them. You can’t touch them. You don’t learn from your mistakes, Riddle, do you?’

_{Deathly Hallows - page 591 - Bloomsbury - chapter 36, A Flaw In the Plan}

I think Harry kind of figured this out on the fly. You'll notice that he does not cast any protective spells after the above dialogue exchange with Voldemort -- because he didn't need to. He understood that he didn't need to anymore, and that he had provided protection to those who needed it.

As to why Harry cast Protego, or any other protective spells, against Avada Kedavra to begin with is not explained in canon. We do know that Harry is a very instinctual individual; perhaps he did what he felt was right, even though perhaps he understood the desire to cast protective spells -- when he knew no protective spell works against Avada Kedavra -- on only a subconscious level.

Answer 2

The shield charm does not just block spells, it (apparently) also fills up space and can be used to push people appart/away. Can't remember quotes right now, though.

Answer 3

The thing is your positing that Voldemort was going to kill them, a valid assumption but still an assumption. You do not know for sure and in the heat of the moment, Harry did not know either, no one but Voldemort knew what spell he would cast.

If you've ever been in the heat of the battle watching friends die, you would know doing anything is better than doing nothing at all. The shield spell is probably the best and only thing he could do for them specifically, until he finished Voldemort.

Answer 4

Harry sucessfully cast the protection spells because the Elder Wand acknowledged him as its real master.

Remember that Draco removed the Elder Wand from Dumbledore while the Professor was casting a silent spell on Harry (Petrificus Totalus). So Draco became the Elder Wand master. When Harry defeated Draco at the Malfoy Manor, the Wand gave him it's allegiance.

And that's why Harry could cast protego on Voldemort's targets, because it won´t act against its own master.

Answer 5

Voldemort was most likely trying to kill them. If he did try to kill them, then Harry's sacrifice would prevent them from dying. However, Harry's sacrifice did not always protect them from other spells, so I think the protego was an instinctual reaction to protect them from other dark spells, like torture. It was also a good distraction obviously because Voldemort looked around for who cast it each time.

I agree that just attacking while under the cloak would have been smarter. But in the Great Hall scene it says Harry couldn't get a clear shot at Voldemort because of all the chaos. Once it was just him and Coldemort it would have been cowardly to not reveal himself and face Voldemort.

Answer 6

Even though you can't block the killing curse you CAN block all other curses and spells. so if someone wanted to use, for example " petrificus tollalus" then it would block it.