His Death Eaters killed Muggles for fun.
When the Dark Lord took power, his supporters killed several Muggles, often for fun.
“Meanwhile, in Gaddley, a Muggle family of five has been found dead in their home. Muggle authorities are attributing the deaths to a gas leak, but members of the Order of the Phoenix inform me that it was the Killing Curse – more evidence, as if it were needed, of the fact that Muggle slaughter is becoming little more than a recreational sport under the new regime.”
- Harry Potter and the Deathly Hallows, Chapter 22 (The Deathly Hallows)
However, though they kill Muggles, they don’t reveal the existence of magic to them. Avoiding revealing this is likely why they didn’t round up and kill larger numbers of Muggles.
“Muggles remain ignorant of the source of their suffering as they continue to sustain heavy casualties,’ said Kingsley.”
- Harry Potter and the Deathly Hallows, Chapter 22 (The Deathly Hallows)
This was also what happened the first time the Dark Lord took power, though he didn’t have the Ministry under his control that time.
“What were You-Know-Who’s supporters up to, levitating Muggles? I mean, what was the point?’
‘The point?’ said Mr Weasley, with a hollow laugh. ‘Harry, that’s their idea of fun. Half the Muggle killings back when You-Know-Who was in power were done for fun.”
- Harry Potter and the Goblet of Fire, Chapter 9 (The Dark Mark)
When he was in power, both times, his followers enjoyed killing Muggles.
Before the Dark Lord took over:
Though the following examples are before the Dark Lord truly took power, they may be relevant. Soon after the wizarding world knows the Dark Lord returned, the Death Eaters kill Muggles in less direct ways, like having a giant attack them.
“That was no hurricane,’ said Fudge miserably.
‘Excuse me!’ barked the Prime Minister, now positively stamping up and down. ‘Trees uprooted, roofs ripped off, lampposts bent, horrible injuries –’
‘It was the Death Eaters,’ said Fudge. ‘He Who Must Not Be Named’s followers. And … and we suspect giant involvement.’
The Prime Minister stopped in his tracks as though he had hit an invisible wall.
‘What involvement?’
Fudge grimaced. ‘He used giants last time, when he wanted to go for the grand effect. The Office of Misinformation has been working round the clock, we’ve had teams of Obliviators out trying to modify the memories of all the Muggles who saw what really happened, we’ve got most of the Department for the Regulation and Control of Magical Creatures running around Somerset, but we can’t find the giant – it’s been a disaster.”
- Harry Potter and the Half-Blood Prince, Chapter 1 (The Other Minister)
They also allow Dementors to attack people, possibly including Muggles.
“And as if all that wasn’t enough,’ said Fudge, barely listening to the Prime Minister, ‘we’ve got Dementors swarming all over the place, attacking people left right and centre …”
- Harry Potter and the Half-Blood Prince, Chapter 1 (The Other Minister)
The Death Eaters blackmailed Fudge into allowing them to blow up the Brockdale bridge by threatening a mass Muggle killing.
“He Who Must Not Be Named has now been joined by those of his followers who broke out of Azkaban in January,’ said Fudge, speaking more and more rapidly, and twirling his bowler so fast that it was a lime-green blur. ‘Since they have moved into the open, they have been wreaking havoc. The Brockdale bridge – he did it, Prime Minister, he threatened a mass Muggle killing unless I stood aside for him and –”
- Harry Potter and the Half-Blood Prince, Chapter 1 (The Other Minister)
Blowing up the bridge itself likely caused several Muggle deaths.
“The bridge was less than ten years old, and the best experts were at a loss to explain why it had snapped cleanly in two, sending a dozen cars into the watery depths of the river below.”
- Harry Potter and the Half-Blood Prince, Chapter 1 (The Other Minister)
During that time, the Death Eaters also tried to put Muggle officials under the Imperius Curse.
“Now, about Herbert Chorley – your Junior Minister,’ he continued. ‘The one who has been entertaining the public by impersonating a duck.’
‘What about him?’ asked the Prime Minister.
‘He has clearly reacted to a poorly performed Imperius Curse,’ said Scrimgeour.”
- Harry Potter and the Half-Blood Prince, Chapter 1 (The Other Minister)
It’s likely they were trying to take hold of the Muggle government as well.