Why did Mr. Ollivander call Voldemort "The Dark Lord" in Deathly Hallows?

Question

He used to use the rather classic "He-Who-Must-Not-Be-Named":

"After all, He-Who-Must-Not-Be-Named did great things -- terrible, yes, but great."
(The Philosopher's Stone, Chapter 5, Diagon Alley)

But then after being freed from captivity in the Malfoy mannor, he uses "the Dark Lord", like Death Eaters (and Snape. Why HE named him so is asked here for those interested.):

“The Dark Lord,” said Ollivander in hushed and frightened tones, “had always been happy with the wand I made him"

[...]

“The Dark Lord no longer seeks the Elder Wand only for your destruction, Mr. Potter."

[...]

"the idea of the Dark Lord in possession of the Deathstick is, I must admit... formidable.”
(Deathly Hallows, Chapter 24, The Wandmaker)

Why did he change his habit and begin using a term primarily used by Death Eaters, even though he was clearly not friendly toward them?

Answer 1

Well, this seems an appropriate question for me to answer.

As Adamant mentioned, Ollivander was "enthralled" by Voldemort.

Even now, having been tortured and imprisoned by Voldemort, the idea of the Dark wizard in possession of this wand seemed to enthral him as much as it repulsed him.

Deathly Hallows, Chapter 24, The Wandmaker

I believe that this was a technical admiration as much as anything. It would be the equivalent of someone who designs bombs for a living seeing their work used for terrible destruction by a violent dictator. They'd be repulsed but also perhaps curious on some level to how the weaponry operated in a real world situation. Ollivander, who had a deep interest in theoretical wandlore, would've been fascinated to see how a wizard as powerful as Voldemort would wield a wand as powerful as the Elder Wand. That interest may shape how he views (and names) Voldemort to some extent.

I also agree with Himarm that the torture he suffered would have imbued Ollivander with a fearful respect for Voldemort. Consequently, he called him by the name Voldemort wanted the wizarding community to use for him.

Ollivander looked stricken.
"He was torturing me!" he gasped. "The Cruciatus Curse...you have no idea..."

Deathly Hallows, Chapter 24, The Wandmaker

Finally, it's not unheard of for non-Death Eaters to call Voldemort the Dark Lord. Although Death Eaters do seem to be the main group that uses the term there's no reason why they have to have a monopoly on its usage. It seems that you could use You-Know-Who, He-Who-Must-Not-Be-Named or The Dark Lord interchangeably without comment. Only using 'Voldemort' itself was unacceptable in mainstream society. Trelawney, for instance, seemed to use it as her default term whenever she made a prophecy.

The Dark Lord lies alone and friendless...The Dark Lord will rise again with his servant's aid.

(Prisoner of Azkaban, Chapter 16, Professor Trelawney's Prediction)

Answer 2

Ollivander seemed to admire Voldemort

“The owner of the Elder Wand must always fear attack,” said Ollivander, “but the idea of the Dark Lord in possession of the Deathstick is, I must admit . . . formidable.”

Harry was suddenly reminded of how unsure, when they first met, of how much he like Ollivander. Even now, having been tortured and imprisoned by Voldemort, the idea of the Dark Wizard in possession of this wand seemed to enthrall him as much as it repulsed him.

—Harry Potter and the Deathly Hallows

He did not use that title when he spoke to Harry in Philosopher's Stone, but he was in polite company. He has always had a sort of admiration for Voldemort, and it is Harry's impression that this did not change entirely after his imprisonment.

Of course, as @Himarm mentions, it is quite possible that he simply picked up the usage during his time imprisoned in Malfoy Manner. I would say, though, that it is fairly unlikely that someone without Ollivander's degree of fascination with Voldemort would pick this up so easily.

Answer 3

I believe its fairly natural for Olivander to pick up this title for Voldemort while he was imprisoned and tortured.

And into his mind burst the vision of an emaciated old man lying in rags upon a stone floor, screaming, a horrible, drawn-out scream, a scream of unendurable agony ...

‘No! No! I beg you, I beg you ...’

‘You lied to Lord Voldemort, Ollivander!’

‘I did not ... I swear I did not ...’

‘You sought to help Potter, to help him escape me!’

‘I swear I did not ... I believed a different wand would work ...’

‘Explain, then, what happened. Lucius’s wand is destroyed!’

After rescue

Ollivander looked stricken.

‘He was torturing me!’ he gasped. ‘The Cruciatus Curse ... you have no idea ...’

‘I do,’ said Harry. ‘I really do. Please get some rest. Thank you for telling me all of this.’

He was imprisoned for close to a year by the Death Eaters being interrogated, tortured, and potentially making wands for the Death Eaters. When speaking to Voldemort it would probably be prudent to use the proper honorifics when dealing with the man who hold your life in his hands.

As we can see by Bellatrix's outburst here, not using proper respect towards Voldemort is a good way to piss her and others off.

“How come Voldemort wants it?”

Several of the Death Eaters let out low hisses.

“You dare speak his name?” whispered Bellatrix.

“Yeah,” said Harry, maintaining his tight grip on the glass ball, expecting another attempt to bewitch it from him. “Yeah, I’ve got no problem saying Vol—”

“Shut your mouth!” Bellatrix shrieked. “You dare speak his name with your unworthy lips, you dare besmirch it with your half-blood’s tongue, you dare—”

— Order of the Phoenix, chapter 35 (Beyond the Veil)