In Philosopher's Stone, Hagrid explains to Harry:
All wizards who turned evil have been in Slytherin.
Later we learn that he was wrong, as Peter Pettigrew was in Gryffindor and has been a Death Eater. Besides Pettigrew,
Are there any other non-Slytherin Death Eaters from Hogwarts?
All the Death Eaters besides Peter Pettigrew are either mentioned as being in Slytherin House, or not mentioned as being in a house at all, as indicated in this list of Death Eaters.
That said, Quirrell was in Ravenclaw, though he was probably not formally a Death Eater.
Hagrid thought that all Death Eaters (and indeed all witches and wizards who went bad) had been in Slytherin, as mentioned in the question:
"Better Hufflepuff than Slytherin," said Hagrid darkly. "There's not a single witch or wizard who went bad who wasn't in Slytherin. You-Know-Who was one."
—Harry Potter and the Sorcerer's Stone
The fact that Hagrid, who was quite active in the Order of the Phoenix, believed that no Dark wizard or witch had come from a house other than Slytherin strongly suggests that all the Death Eaters he was aware of had been Slytherins, which suggests that those Death Eaters whose houses were not given were probably Slytherins. Given that Slytherin is a house that rewards ambition (surely a common characteristic of Death Eaters), and had an environment that more or less actively fostered pureblood supremacist ideals, it is perhaps not so surprising that (nearly) all Death Eaters came from Slytherin. Of course, the vast majority of Slytherins still were not Death Eaters.
In response to a question about why Slytherin remains a House at Hogwarts, J.K. Rowling says that there are people connected with Death Eaters in the other Houses as well.
ES: But there aren't a lot of Death Eater children in the other houses, are there?
JKR: You will have people connected with Death Eaters in the other houses, yeah, absolutely.
- The Leaky Cauldron interview (July 16, 2005)
She doesn’t state explicitly that there are actual Death Eaters in the other houses, and it’s possible that there are only people connected with them but no one who actually joined them in other houses, but it seems likely from this that there would be Death Eaters in the other houses as well. People mostly, though not always, end up in the same house as their family.
In addition, we don’t know the houses of many of the Death Eaters, even the named ones. Barty Crouch Jr, Rookwood, Yaxley, and Rowle are some of the Death Eaters whose houses are never stated or implied.
We do know that there are many Death Eaters, some that we don’t even know, like a few mentioned in the Half-Blood Prince. We cannot confirm that all of the numerous Death Eaters are Slytherin. If there is a Death Eater in Gryffindor, which is the enemy house to Slytherin, there ought to be Death Eaters in Hufflepuff and Ravenclaw as well. Remember, Quirrel was evil and in Ravenclaw, and in the Cursed Child, Cedric became evil.
We only really know of a few Death Eaters who were Slytherins actually. Snape, Avery, Bellatrix and Rodolphus Lestrange, Evan Rosier, and Wilkes were said to have been part of a gang of Slytherins in his day at Hogwarts. Lucius and Narcissa were said to have been Slytherins as well as Regulus Black. Mulciber was implied to have been part of the above gang by Lily Evans. We can probably conclude that Rabastan Lestrange was one too. Crabbe, Goyle, and Nott are likely due to their sons being Slytherins as well. Probably most of the early Death Eaters were Slytherins too. The houses of everyone else are up in the air, though probably at least 2/3 are Slytherin. Personally I've always pictured Rookwood and Barty Crouch as Ravenclaws as they both seem highly studious and were given surprised reactions by the public when their allegiances were outed.
