They mention in the books that Grindelwald was the "Dark Lord" before Voldemort, and that Dumbledore was the one to stop him by taking the Elder Wand in a fair duel.
What I couldn't find was the specifics on the rise of the dark wizard Grindelwald, and the devastation he caused. So here goes with my question:
How bad (numerically by deaths) was Grindelwald's reign of terror compared to Voldemort's?
After much researching I haven't found an exact figure related to how many people Grindelwald killed or even imprisoned in Nurmengard. However, there was a line from wikia - Gellert Grindelwald
He had absolutely no complaints about the murder of hundreds of innocent people who, from his perspective, were lesser than he
This, however, doesn't confirm explicitly that he murdered them himself, potentially his followers could have committed these crimes. Yet if it was him it does at least offer some indication as to how many people he killed.
To reinforce the above quote there is also another from Dumbledore:
"People were dying and he seemed unstoppable, and I had to do what I could." —Albus Dumbledore
This does confirm as we know that people were dying under Grindelwald's reign of terror. Albeit not offering up a figure.
In interviews with JKR there are hints and suggestions, as well as being noted by other people who have answered this question that Grindelwald had been one of Hitler's allies. Thereby potentially placing the number of deaths that he was involved in into millions.
Grindelwald terrorised all of Europe
There is evidence that indicates that Grindelwald wished to be more of a ruler than a murderer.
Firstly
He was not a wanton killer or torturer, but he and his followers (in a single-minded and unpopular quest; one's allies must often be unsavoury characters) committed numerous crimes, including several known murders and eventually genocide.
Secondly
his decision to stun Gregorovitch, not kill, when he acquired the Elder Wand hints at a willingness not to kill simply as a matter of course
Thirdly
Grindelwald merely wanted to dominate and rule Muggles, whereas Voldemort wished to kill them outright, along with anyone he considered not of pure blood
Fourthly
He succeeded in creating a prison, Nurmengard, to serve as a form of containment for his enemies...Though he certainly wasn't as blatantly murderous as Voldemort
Grindelwald would have been the evilest wizard ever, except that a generation later, U-No-Poo arrived to steal his crown.
As we don't know exactly how many people died in that time, we can't say how many people died because of Gellert.
Voldemort was really 'evil', and killed those who apposed him, whereas Grindelwald was acting 'for the greater good', and built a prison for his enemies.
Gellert Grindelwald has not been given enough importance in the series as his criminal activities were mostly in the past as compared to the timeline of Harry Potter and also not in Britain where most of the story was. So comparing his dark reign with that of Voldemort is a difficult task which deviates mostly from canon and makes way for certain assumptions, but I will try to do the job from the information we have from canon.
Grindelwald is a highly skilled wizard, compared to Dumbledore as an equal at the age of 17 and is known to be expelled from Durmstrangs for experiments he conducted,
Educated at Durmstrang, a school famous even then for its unfortunate tolerance of the Dark Arts, Grindelwald showed himself quite as precociously brilliant as Dumbledore.
...
At sixteen years old, even Durmstrang felt it could no longer turn a blind eye to the twisted experiments of Gellert Grindelwald, and he was expelled.
-Harry Potter and the Deathly Hallow's, Chapter 18:The Life and Lies of Albus Dumbledore
Grindelwald had plans to establish Wizard ruling on Muggles even from his school and he shares his ideas with Dumbledore when they meet each other at Godric's Hollow.
Gellert ---
Your point about Wizard dominance being FOR THE MUGGLES’ OWN GOOD --- this, I think, is the crucial point. Yes, we have been given power and yes, that power gives us the right to rule, but it also gives us responsibilities over the ruled. We must stress this point, it will be the foundation stone upon which we build. Where we are opposed, as we surely will be, this must be the basis of all our counterarguments. We seize control FOR THE GREATER GOOD. And from this it follows that where we meet resistance, we must use only the force that is necessary and no more. (This was your mistake at Durmstrang! But I do not complain, because if you had not been expelled, we would never have met.)
Albus
-Harry Potter and the Deathly Hallow's, Chapter 18:The Life and Lies of Albus Dumbledore
The ruling of Grindelwald had shown signs of Dumbledore's suggestions being implemented like having the slogan FOR THE GREATER GOOD and carving it on his entrance to prison Nurmengard, which he had build to hold his opponents.
“I[Hermione] think that's the worst bit. I know Bathilda thought it was all just talk, but 'For the Greater Good' became Grindelwald's slogan, his justification for all the atrocities he committed later. And . . . from that . . . it looks like Dumbledore gave him the idea. They say 'For the Greater Good' was even carved over the entrance to Nurmengard.”
-Harry Potter and the Deathly Hallow's, Chapter 18:The Life and Lies of Albus Dumbledore
Grindelwald's ruling was primarily dictatorship, he wanted to make Muggles believe that Wizard ruling will earn fruitful results and wanted to discard Statute of Secrecy, he was always compared with Adolf Hitler and alexwlchan's answer here draws an excellent similarities between them. Also this essay here.
Compared to Voldemort, he had enough kindness to build a prison for his opponents rather than just killing them with the Death Curse and he is not too kind to have morals for not killing them,
Krum’s jaw muscles worked as if he were chewing, then he said,
Grindelvald killed many people, my grandfather, for instance. Of course, he vos never powerful in this country, they said he feared Dumbledore
-Harry Potter and the Deathly Hallow's, Chapter 8:The Wedding
It's not new for a Dark Wizard to be fearful of Dumbledore, but he is the first of his kind.
Even though JK Rowling commented that Wizard wars and Muggle wars being interconnected, throwing all the deaths of World War II into Grindelwald's account is absurd.
Voldemort's killings will outnumber that of Grindelwald's because of his vast number of Muggle killings for Inferi, which Grindelwald had no mention about in canon. The number of people involved in Inferi are estimated by Slytherincess here.
So, through canon it is difficult to estimate the amount of killings Grindelwald committed because he is detached with the Britain community because of his fear in Dumbledore.
Possibly, 50-80 million casualties
If Grindelwald was connected to World War II -- and according to this answer he is --, then we are talking about millions.
World War II fatality statistics vary, with estimates of total dead ranging from 50 million to more than 80 million.
(source: Wikipedia)
I'm assuming, of course, that Grindelwald was a key-figure in this conflict -- not a coincidence that WW II ended by the time he was defeated.
(and, btw, why fight with just a hundred wizards army, when you can have hundreds of millions of disposable Muggles with you?)
One of many possible scenarios: Hitler is rumored to have an interest in the paranormal. If that was the case, he would have loved to count with magical items, divination or a wizard-squad to help him. Easy to see Grindelwald using the chance to become his right-hand -- or even the one pushing the paws after a while.
[oh, the irony: millions of Muggles fighting among themselves when a wizard is the one pushing the paws. Not so different from half-blood Riddle using Purebloods prejudices for his own agenda...]
PS: even if Grindewald was not a key-figure in WW II, a global wizard war is bound to create much more damage than a single year (or less than that) of dominance restricted to the British soil. So, I would say that Grindelwald's reign was much worse than Voldemort's.
