If I was them (and in a way I am) I’d probably kill the witnesses and bail.

So I totally get this conclusion, but I think it’s worth slowing down and considering whether this makes as much sense as it seems at first glance. The fact that magic exists means that simply killing someone simply doesn’t do much to shut them up, if a sufficiently powerful entity is willing to spend the resources. The fact that undeath exists means that killing someone has a very real risk of making them a bigger threat than they were in life–it’s not like you can just stab a ghost. Cultists, being familiar with eldritch powers themselves, know this full well. This means that keeping people merely out of communication might be the simplest way to achieve their actual goal with the minimum of fuss. They don’t need someone quiet forever, they just need enough secrecy to achieve their goals. Murdering every person who takes an interest in them is mission creep.

Also, keep in mind cults generally exist for specific purposes, and people join them for specific purposes. These purposes aren’t necessarily overtly evil at the rank-and-file level, which is integral to their recruitment. The turnip farmer who wants to resurrect a dead harvest god to grow more turnips might be okay with some dodgy rituals the church wouldn’t approve of, but straight up committing multiple murder might take some working up to, if he can be talked into it at all.

So in short, consider what your cult wants, and assume a degree of rationality and thoughtfulness (at least, when they’re not channeling horrors from another plane). What do they want, and how the party could provide what they want?

If they’re really lucky, they’ll end up working for the Laundry only once. Residual Human Resources is a bad way to go out.

Charles Stross’ Laundry series is basically this concept set in the present day: magic is a branch of mathematics, which means it can be computed and programmed.

It is perhaps worth noting at this point the series genre is cosmic horror.

For safety reasons, a magical museum might well want to avoid putting actual items on display, especially since Illusion magic makes it trivial to create simulated substitutes crowds can safely interact with. This also keeps security budgets manageable, since even a magical museum probably doesn’t have the kind of money to protect the arcane equivalent of a small nuclear arsenal.

So the museum researchers might be happy keeping items on indefinite loan, as long as they know the borrower and have guaranteed access when they need it. This way the mage owning the item is responsible for keeping it safe, and arcane historians–who definitely didn’t spend all those years in magic academy just to play guard–don’t have to.

As a bonus, this method gives the party a quirky but knowledgeable NPC contact who can give them clues or set up sidequests whenever you need.

I’ll preface this by noting that the sin of sloth has traditionally been understood to be a sin of omission, not just commission, i.e., you are insufficiently devoted to the things you ought to be.

Which means you could, in theory, have a (reflavored tiefling) devil paladin so devoted to sloth he works against evil causes. He’s not interested in good per se, it’s just that advancing the interests of good and traveling with a good adventuring party has the best ROI for failing to carry out his evil responsibilities.

Naturally, this has caused a fair amount of controversy among sloth devils, and there is a multi-century trial going on in the Hells about whether this ought to be allowed. This is not expected to be resolved in the foreseeable future because the advocates for both parties keep filing their responses well after petition deadlines expire.