I believe that the only way to kill someone so that they cannot be revived or cured is with Avada Kedavra .
It also has no counter-curse, so you cannot deflect it.
However, without immediate medical attention, there are plenty of other things that would kill wizards or witches.
You could cut their throats ( Sectumsempra ), throw stuff at them that breaks bones, poison them, etc.
A skilled mediwitch or mediwizard could cure them if they got treatment in time, but they're not going to be getting that treatment during the middle of a battle.
There are probably also spells that have counters to nullify them -- but that would require someone to cast that nullifying spell.
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From the wikia page, This battle ended in the death of Bellatrix, who continued laughing derisively at the efforts of her opponent until the moment she realised Molly had defeated her.
Hence, it can be clearly inferred that Bellatrix treated Molly as a weak opponent and her overconfidence led her to death.
On the other hand, Molly was highly angered as Bellatrix was torturing Ginny at that moment and the motherly love gave her the courage to face a death eater.
I can see only psychological reasons behind Bellatrix's death as it was difficult to overpower her when she was in a sane state of mind.
An extract from the wiki page: ... fires a curse that hits Bellatrix right over the heart, killing her.
Though the exact curse used is unknown, I believe she used a legal curse(such as the Stunning curse) and the placement directly over the heart is what killed Bella.
If you remember back to OotP, someone (perhaps Madam Pomfrey) remarks that it was a wonder that McGonagall did not die from so many Stunning spells.
So, perhaps it is possible for one to die from a well placed Stupefy or some other type of Stunning Spell, and this is what I believe happens here.
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Avada Kedavra is a sure way of killing someone, and illegal, but is hardly the only way someone can be killed using a spell.
There are a few examples of this over the series: When Filch's cat is assumed dead inBook 2
, Lockhart names some obscurespell (the *Transmogrifian Torture)as the one to have killed her.
When McGonagall is attacked by Aurorsin Book 5, Hermione says that she washit straight in the chest and she'snot young, showing how even a stunnermay cause damage.
Avada Kedavra is highly illegal because - It's meant only to kill.
It has no counter.
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The book doesn't specify what spell was used.
I see no reason to rule out Avada Kedavra though.
The curse killed Bellatrix almost instantly which implies Avada Kedavra.
The book also notes that Jets of light flew from both wands, the floor around the witches' feet became hot and cracked; both women were fighting to kill.
(emphasis added) Molly was outraged that Bellatrix was attacking her daughter; everybody was under the impression Harry was just murdered (he hadn't made his reappearance yet).
Could it have been something other than Avada Kedavra?
Of course - but I see no reason to rule it out especially given the circumstances.
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The answer does lie in the fact that death by the brute force of other spells must have done the job.
In the movie, it seems Molly hit Bellatrix with a Petrificus Totalus (body binder) rendering her absolutely and totally vulnerable and the force of a general explosive spell like bombarda or reducto gave her the works.
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In the book, the author did not specify the curse used.
However, in the film, if you'll take a close look at Molly Weasley's wand (watch a clip of it on youtube), the light coming from it is green, which means, Avada Kedavra is being cast nonverbally, and the reason for Bellatrix's body to tear into pieces might be reducto , which is being casted nonverbally.
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Molly DID use Avada Kedavra
This question was just answered by Pottermore today:
There’s no dearth of magical talent among the Weasley brood, but Molly and Ginny each have impressive and specific magical skills.
Molly’s, of course, tie in with her own personal preferences – so she’s not only marvellous at the magical assembly of feasts fit to feed five hundred
, she’s also a dab hand at the killing curse if there’s a witch-gone-wrong in need of offing.
Avada Kedavra is the only spell referred to as " the killing curse " ‘Ah,’ said Moody, another slight smile twisting his lop-sided mouth.
‘Yes, the last and worst.
Avada Kedavra ...
the killing curse .’
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Molly used Petrificus Totalus, the body binder spell, hence the fact Bellatrix was frozen, then she used a strong exploding spell, probably Reducto or Diffindo, whch destroyed Bellatrix.
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I believe that Molly used some sort of body binding curse, but it could have been a curse of her own creation, or simply a spell
she did not even know and cast out of anger,
the spell that made her explode was most definitely reducto and not diffindo .But
the correct answer has not been stated by the author, so we shall never know for sure.
scifi.stackexchange.com/users/4306/Blair
From what I saw in the movie, it seems that Molly hit Bellatrix with Petrificus Totalus, and then finished her off with Reducto.
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It seems that the only officially known way to instantly kill someone with a curse was "Avada Kedavra".   So, (spoilers):    considering that was definitely NOT what Molly Weasley used to kill Bellatrix Lestrange at the end of Battle for Hogwarts, how did she kill Bellatrix?
How was this character killed without Avada Kedavra?
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<harry-potter><spells>
scifi.stackexchange.com/questions/4120
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One theory is that as in the film, the light coming from Molly's wand  is green,  Avada Kedavra  is being cast nonverbally. However, the reason for Bellatrix's body tearing into pieces might be the use of  reducto.  Another theory is that Molly hit Bellatrix with a Petrificus Totalus (body binder) rendering her absolutely and totally vulnerable, then finishing with the force of a general explosive spell like bombarda or reducto. A legal curse such as the Stunning curse may have been used, placed directly over the heart. However, this is not specified in the book. Although the book does not specify what spell Molly Weasley used to kill bellatrix Lestrange, many believe it was the killing spell, Avada Kedavra. Alternatives to this include Petrificu Totalus with bombarda or reducto.
Avada Kedavra seems the most likely curse used by Molly Weasley to kill Bellatrix Lestrange. Molly may have used other curses to kill Bellatrix, such as Petrificus Totalus and bombarda or reducto. The book does not specify what spell was used. 