Why do people still refer to Voldemort as "Lord"?

Question

I'm aware that Voldemort named himself, as an anagram of Tom Riddle, so Lord fits, however why do people still use this? Lord is a title, given to, well... lords.

I know Harry and Dumbledore often call him Tom to try and "re-humanise" him, but why do others, seemingly scared wizards/witches use the title of "Lord" when his name is spoken? Is it simply fear of angering him?¹

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¹ In the film portrayal of Order of the Phoenix, Harry refers to him as Lord Voldemort during Doleres Umbridge's first class.

I have not read the books, so it could be there is a canon reason that is not obvious in the films, if not, I will accept a well-reasoned, out-of-universe answer.

Answer 1

I can think of a couple of reasons.

We know he has the ego for it, plus by the time he graduated he had already opened the chamber of secrets and proclaimed himself the heir of Salazar Slitherin, who is never exactly stated to have a title, but is enormously significant regardless.

That plus his power and skill and ideology attracted to him a fairly large group of followers, the death eaters, who are totally obedient out of fear at worst, and completely fanatical worshipers at best. Add to that his many past and present relationships with various factions of magical creatures.

He makes a pretty good case for himself. The title of "Dark Lord" has never really been completely rooted in the traditional definition of inherited landownership and Nobility.

It makes perfect sense that the death eaters would call him that, and refer to him as lord to other non death eaters. And the name stuck out of sheer public terror at the awful things he and his followers did.

"You don't know who his supporters are, you don't know who's working for him and who isn't; you know he can control people so that they do terrible things without being able to stop themselves. You're scared for yourself, and your family, and your friends. Every week, news comes of more deaths, more disappearances, more torturing... the Ministry of Magic's in disarray, they don't know what to do, they're trying to keep everything hidden from the Muggles, but meanwhile, Muggles are dying too. Terror everywhere... panic... confusion... that's how it used to be." -Sirius Black

Answer 2

It must be a 'life peerage' - that would require an Act of Parliament to remove.

Archer was sent to Belmarsh Prison, a Category "A" prison, but was moved to Wayland Prison, a Category "C" prison in Norfolk, on 9 August 2001. ...

On 21 July 2003, Archer was released on licence, after serving half of his sentence, from Hollesley Bay. Archer was allowed to keep his peerage as it is a life peerage rather than "an honour under the Crown". An Act of Parliament would be required to enable such a change to someone's peerage.

Wikipedia: Jeffrey Archer # Prison

Presumably he retained his seat in the House of Lords:

... an act of parliament is required to revoke a peerage.

Andrew Mackinlay, Labour MP for Thurrock, has tabled a Commons motion calling on the government to legislate to disqualify peers imprisoned for criminal offences from sitting in the Lords. The Archer saga endorsed the case for an elected second chamber, he said.

"Not only is it an undemocratic place, but you could commit murder and when you get out of prison you could still go back into the House of Lords," he said. ...

"If a member of the House of Commons had been sent to prison for a similar crime yesterday he would have to forfeit his seat. "Why do we extend to the glitterati, the rich and the powerful, all those people in the magic circle which runs this country, the privilege of going back to parliament after they have come out of prison?"

Disgraced Archer may lose peerage, The Guardian, 20 July 2001

I'm not sure how the law has now changed, but I think the article above reflects the law in the late '90s, when the Potter saga is set.

The situation would be much easier if he were 'Sir Thomas Voldemort':

Mr Goodwin lost his honour for “services to banking” despite not having been convicted of any criminal offence nor being professionally censured, the normal requirements for annulling a knighthood, CBE, OBE or MBE.

Fred Goodwin is shredded: former RBS boss stripped of knighthood, Robert Winnett and Rowena Mason, The Telegraph, 31 Jan 2012

^{Alternatively, it could just be to make the apophasis clearer. If you say "The Dark Lord" then it's a bit clearer who you mean than if you say "you-know-who" or "thingummy-bob".}