Snape sort of might be - he joined the Death Eaters but defected.
Snape joined the Death Eaters, despite being a half-blood born to a Muggle father and witch mother. It’s unclear how much of his desire to join the Death Eaters was because of their beliefs about pure-bloods, and how much of it was due to his interest in the Dark Arts, so it’s hard to tell whether he was truly a pure-blood supremacist or if his reason was more his interest in Dark Arts.
“I was going through the rest of the old Prophets and there was a tiny announcement about Eileen Prince marrying a man called Tobias Snape, and then later an announcement saying that she’d given birth to a –’
‘– murderer,’ spat Harry.
‘Well … yes,’ said Hermione. ‘So … I was sort of right. Snape must have been proud of being “half a Prince”, you see? Tobias Snape was a Muggle from what it said in the Prophet.”
- Harry Potter and the Half-Blood Prince, Chapter 30 (The White Tomb)
In addition, Snape defected from the Death Eaters and spied on them for Dumbledore, so his allegiance actually being to them or their cause was only for a fairly short amount of time. Snape seems like a ‘partial’ example for these reasons, but he’s provably a half-blood.
Many Death Eaters’ blood status isn’t said, likely there are more.
There are many Death Eaters whose blood status isn’t mentioned. It’s likely that there are other half-bloods who’ve joined the Death Eaters, and more broadly, more half-bloods who believe pure-bloods are superior, but not everyone’s blood status is stated. It’s likely that at least some of the Death Eaters are half-bloods, because pure-bloods are fairly rare. Hermione speculates that most Death Eaters are probably half-bloods, but she’s of course a bit biased since she’s Muggle-born and would be glad to think the Death Eaters are hypocritical and don’t live up to their own ideals. Also, she’s clearly taking a swipe at Ron and Harry, and it’s certainly an emotional statement.
“The Death Eaters can’t all be pure-blood, there aren’t enough pure-blood wizards left,’ said Hermione stubbornly. ‘I expect most of them are half-bloods pretending to be pure. It’s only Muggle-borns they hate, they’d be quite happy to let you and Ron join up.”
- Harry Potter and the Half-Blood Prince, Chapter 12 (Silver and Opals)
Even if Hermione is wrong about most Death Eaters being half-blood, it’s highly likely that at least some of the Death Eaters are actually half-blood, especially since pure-bloods are rare.
There are half-bloods in the Malfoy family, they might count.
Though no one in the Malfoy family ever married a Muggle or Mudblood, some of them did marry half-bloods. Since they’re likely to marry people who agree with them, those half-bloods are more likely than not to themselves believe that pure-bloods are superior.
The Malfoys’ unfeigned contempt for all Muggles who could not offer them jewels or influence, and for the majority of their fellow wizards, drew them naturally towards the pure-blood doctrine, which seemed for several years in the twentieth century to be their likeliest source of untrammelled power. From the imposition of the Statute of Secrecy onwards, no Malfoy has married a Muggle or Muggle-born. The family has, however, eschewed the somewhat dangerous practice of inter-marrying within such a small pool of pure-bloods that they become enfeebled or unstable, unlike a small minority of fanatic families such as the Gaunts and Lestranges, and many a half-blood appears on the Malfoy family tree.
- The Malfoy family (Pottermore)
Therefore, it’s likely the Malfoy family has more half-bloods who believe in pure-bloods’ superiority. In addition, children born to pure-bloods and half-bloods are half-blood. So any children who were born to these half-bloods who marry into the Malfoy family, who would likely be raised to share the family values, would also then be half-bloods who believe that pure-bloods are superior.