Okay, there's some great ideas already, so I'm going to swerve a bit and ask some questions to see if there's an in universe reason for things.

Your assumption is that the cultists would kill the prisoners and flee.

Is that because of the cult's beliefs? If not, then there's no reason to start off from evil = kill as the first choice. You don't have to be Stupid Evil just because you're evil. What god or entity are they serving? What's the "portfolio" of the entity? What is their long term goal?

Depending on those answers, it's possible to cook things up that don't break the world building, or rely on deus ex machina to save them.

As an example, I had a similar situation with an organization in my home brew world/system. A cabal of political and social manipulating villains captured part of the group because the players just did something silly, they didn't even fail a roll until they were trying to escape, and it was already too late anyway, they would have been followed.

But, this organization was a secret organization. Disappearing people was definitely a possibility, but not when they didn't catch everyone. So the minions of the organization rendered the captured people unconscious, then used memory wiping spells along with drugs to cover their tracks more fully. They dumped the party members off near a hospital, and "sanitized" the location they were at before the party came along. Now, the leadership of the organization were rightfully pissed, and took steps for revenge that would be much easier to cover up, but they didn't make a rash decision in the moment so that they could make it clean.

It didn't work out for the organization, the party did take them down eventually, but it was a much more interesting fight along the way because the players were invested more.

Not saying that's the right answer for your story and table, it's just an example of the kind of thinking that can be done in universe.