In Deathly Hallows, why do Voldemort & co just tie Hagrid up? Why not kill him?

Question

Towards the end of Deathly Hallows, when Harry goes to BigV in the forest, we see Hagrid tied to a tree on the edge of the clearing. They even use Silencing Charms on him to shut him up.

Voldemort killed his own people for hearing about the cup or just being annoying, yet spares one of Harry's closest friends?

Why would they keep him alive?

What possible use could he be to them? A lure for Harry? Keeping the beasts of the forests at bay perhaps?

Given what we know of Riddle, I would expect him to order the Death Eaters to "kill Potter's friends, the more the better" and not take hostages!

Answer 1

I suspect there may not be any canon answer to this. But here goes my take...

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In-Universe

As a Witness
It is most likely that Voldemort was making sure that there was a non-Death Eater witness on hand to relay the death of Harry Potter and the circumstances surrounding the death to Harry's supporters.

Psychological Warfare
Another point as mentioned by DVK is to add an element of psychological damage to his enemies.

Voldemort truly believed in defeating his enemies, not only by physical means, but also using psychological means:

[Voldemort]: no man alive can threaten me now! Watch! Crucio!”

Harry had been expecting it, knew his body would not be allowed to remain unsullied upon the forest floor; it must be subjected to humiliation to prove Voldemort’s victory.
^{-Harry Potter and the Deathly Hallows, Chapter Thirty-Six (The Flaw in the Plan).}

Hagrid is impeccably strong, as well as a fierce and unrelenting member of the Order of the Phoenix. The message of "look, we've got your fiercest warrior and he is nothing but a puppet to us now!" is surely to strike despair into the hearts of his enemies; as we can see has happened to Prof. McGonagall:

... “NO!”

The scream was the more terrible because he had never expected or dreamed that Professor McGonagall could make such a sound. He heard another woman laughing nearby, and knew that Bellatrix gloried in McGonagall’s despair. ...
^{-Harry Potter and the Deathly Hallows, Chapter Thirty-Six (The Flaw in the Plan).}

Note that Voldemort was definitely not expecting to capture Hagrid and use him in this way, but he saw the opportunity present itself and used it to his advantage. As we see that he (Voldemort) was originally thinking of dragging Harry's body back to the survivors, but changes his mind when he realises that using Hagrid would have a greater impact:

“Now,” said Voldemort, “we go to the castle, and show them what has become of their hero. Who shall drag the body?

No — Wait —”

There was a fresh outbreak of laughter, and after a few moments Harry felt the ground trembling beneath him.

“You carry him,” Voldemort said. “He will be nice and visible in your arms, will he not? Pick up your little friend, Hagrid. And the glasses — put on the glasses — he must be recognizable —”
^{-Harry Potter and the Deathly Hallows, Chapter Thirty-Six (The Flaw in the Plan).}

Kill With Meaning
You are incorrect in your assumption; Voldemort doesn't just kill for fun, or for no-good-reason at all.

He is extremely meticulous and cunning, and knows when to kill as well as when not to kill, as I've pointed out here:

One simple movement and the child would never reach his mother . . . but unnecessary, quite unnecessary. . . .
^{-Harry Potter and The Deathly Hallows, Chapter Seventeen (Bathilda's Secret).}

Voldemort does not just want to annihilate the entire world along with the whole wizarding population. He is especially against the spilling of "pure-blood", but also knows that he must 'rule not kill':

“The battle is won. You have lost half of your fighters. My Death Eaters outnumber you, and the Boy Who Lived is finished. There must be no more war. Anyone who continues to resist, man, woman, or child, will be slaughtered, as will every member of their family. Come out of the castle now, kneel before me, and you shall be spared. Your parents and children, your brothers and sisters will live and be forgiven, and you will join me in the new world we shall build together.”
^{-Harry Potter and the Deathly Hallows, Chapter Thirty-Six (The Flaw in the Plan).}

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Out-of-Universe

Right in the feels
JKR seems to be very fond of symbolism in her story. When we first meet Harry as a baby, he is carried in by (non-other than) Hagrid himself! and now Hagrid has to carry Harry's (apparently) dead body back to the throngs of onlookers.
(It truly made my heart melt when I read that part)

Answer 2

There's no canon info for what Voldemort was thinking; but it's possible it was for psychological value. (same as showing defeated and dead Potter to Hogwarts defenders). It'd be too much work to carry a dead Hagrid all the way out of the forest.

Answer 3

First, Hagrid is perceived as a blundering fool by Voldemort (recalling the time of Tom Riddle and Hagrid in Hogwarts), therefore, considers the half-giant not dangerous. The Dark Lord renders Hagrid immobile to avoid any complications.

Second, Hagrid is very close to Harry. The inflicted pain would be psychologically most debilitating if Hagrid can be used as a pawn in the transaction of Harry. Finally, Hagrid is a half-giant. Giants were a part of the Dark army. There may be some considerations involved here.

In conclusion, I think Hagrid is not killed by Voldemort on account of its sheer irrelevance to the big plans of Voldemort.

Answer 4

Killing Hagrid needlessly wouldn’t help get the rest to surrender.

When the Dark Lord had Hagrid carry Harry, his intention was to show those fighting him that their Boy who Lived is dead, and hopefully convince them to surrender to him. He was attempting to get those who had been fighting him all along to willingly stop fighting and submit to his rule.

“Harry Potter is dead. He was killed as he ran away, trying to save himself while you lay down your lives for him. We bring you his body as proof that your hero is gone.

‘The battle is won. You have lost half of your fighters. My Death Eaters outnumber you and the Boy Who Lived is finished. There must be no more war. Anyone who continues to resist, man, woman or child, will be slaughtered, as will every member of their family. Come out of the castle, now, kneel before me, and you shall be spared. Your parents and children, your brothers and sisters will live, and be forgiven, and you will join me in the new world we shall build together.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)}}

Killing Hagrid at that time would have served him no particular purpose - Hagrid only fought him as much as the others who the Dark Lord was still willing to let live. In addition, the Dark Lord would have wanted the others to feel that if they surrender then, he wouldn’t kill them, since that’d make them more willing to do it than if they think they’ll probably die either way. Though Hagrid wouldn’t count as ‘magical blood’ like the others since he’s half-giant, killing him was still unnecessary and would do nothing to inspire confidence that surrender ensures safety. It would be pointless and counterproductive to the Dark Lord’s actual goal of getting the wizards fighting him to submit.

Out-of-universe, J.K. Rowling had always planned that Hagrid would carry Harry.

From an out-of-universe perspective, J.K. Rowling had always pictured Hagrid surviving to carry Harry out of the forest, and had planned it for quite a while.

VO: It has to do with Hagrid. The loveable, giant-sized Gamekeeper.

MV: A lot of people were worried that Hagrid would die.

JKR: Yeah.

MV: Was that ever a plan?

JKR: Yes ... Everyone was up for grabs. Everyone. But actually from very early on ... I wanted Hagrid to be the one who carried Harry out of the forest. That had been planned for so long. And I wanted Hagrid to believe that Harry was dead, so that-- I had that image in my mind that Hagrid would survive to do that, to carry him out.
_{^{- Dateline (July 29, 2007)}}

So, she’d already planned he’d survive and be the one who carries Harry when he’s thought dead.

Answer 5

Because, well, JKR just wasn't up for it.

http://tvtropes.org/pmwiki/pmwiki.php/Main/WhyDontYouJustShootHim?from=Main.Whydontyajustshoothim

Answer 6

The killing curse is expensive in terms of mana per pound of mass, plus a giant's magical permeability is at least two orders of magnitude greater than normal, so we can expect a half-giant's to be somewhere in the middle. Now, Voldemort does not need to do the calculations in his head to see that he needs to save his energy in this case.

JK doesn't mention all the technical details for good reasons, it might come in handy when Harry goes to the University of Magic in Bulgaria.