This is really interesting! So, now that all the primaries are out, why would the different houses gravitate towards joining the Death Eaters? Like, why do the Death Eaters pull more Slytherins than Hufflepuffs?
Well our Primaries don’t always match up with what people tend to be sorted in-canon, and when they do it’s not because we intended them to! So to say that Slytherin Primaries would gravitate more than the other houses toward being Death Eaters, I would actually say is probably not the case.
You’re right in thinking that Hufflepuffs have a lot of potential to become Death Eaters. We sort Bellatrix Lestrange as a Hufflepuff Primary. If they can be convinced that muggles and muggle borns are inhuman, or at least less human than wizards and witches, then the actions of the Death Eaters probably wouldn’t carry the same level of horror for them. Ravenclaws would join the Death Eaters if they accepted the system that Voldemort proposed, and Gryffindors could be swayed by twisted appeals to their morals. Get their logic or their gut to follow the teachings of Voldemort, and you’ve got yourself some new Death Eaters. I really don’t think that in the face of something as plainly evil as Voldemort, you’re going to have any particular house favoring the cause.
A big part of the why then, in the case of the books, is that…JKR really doesn’t like Slytherin. Her Slytherins are all evil. The most redeemable Slytherin we probably have is Slughorn (Slytherpuff, we think), who’s still unsavory. And as far as I can tell, her justification for that is that power corrupts. And arguably Narcissa when she lies to Voldemort to save Draco, but…she’s still married to and supportive of a Death Eater. She doesn’t get to e considered a good person. According to how JKR writes them, all Slytherins get caught up in their ambitions and their schemes and they forget the things that really matter— bravery, kindness, friendship. Things that she doesn’t particularly show Slytherins being capable of.
And if you take people like that and put them in a war-torn world where one of the ways out, one of the ways to protect your family, is to join with Voldemort? I can understand the train of logic that would get you there. And it’s more of a stretch, but I can understand the thematic logic, at least, that would get you to the final battle and not have a single Slytherin student wanting to fight.
There are some other interesting theories, like the importance Slytherin puts on blood purity encouraging and propagating and being at the least sympathetic to Voldemort’s violence, and like the social importance put on your Hogwarts house and the pressures that would go along with that, but none of those would explain why Slytherins are unanimously awful. The story of how ambition and selfishness and family loyalty can be positive things just wasn’t the story that JKR was interesting in telling.
Which like, yeah okay, they’re her books and she can do what she wants with them.
But as far as your question? I don’t think Slytherins are more likely than the other houses to turn Death Eater. I could come up with some answer about protecting your family by siding with whoever the biggest bad in the room is, but I don’t actually think that’s a strong enough motivation for most people in that situation. You’re going to end up with Slytherins who flee and Slytherins who stay quiet and who join the Ministry and try to help from political angles, and you’re going to end up with Slytherins who roar right alongside those Gryffindors and join the fight with all their gusto.