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?