I adapted the hypothetical to fit that conversation.
So is the hypothetical you are inviting us to consider this one?
1. The GM tells the players they come to a left/right "fork". The players choose one or the other. Whichever they choose, the GM tells them that their PCs come upon the cultists about to sacrifice the prisoners.
Or is it this one?
2. The GM tells the players they come to a left/right "fork". If the players choose the right, their PCs will come upon the cultists about to sacrifice the prisoners. If they choose the left, their PCs will come upon something else (let's say, the office/library); and if, following that, the PCs then go down the right path, they will come upon the cultists about to sacrifice the prisoners.
My view of (1) is that the whole thing looks like poor GMing (with a possible exception - see below). Why is the GM writing in a choice of paths, if in fact the fiction is going to be constant? Maybe there is a story to tell about the choice adding colour? I don't think I've ever heard of anyone doing (1) in D&D, which tends to be somewhat map-oriented.
The exception I can think of is if the GM is anticipating a chase/flight scene - and so expects the players to be keeping track of their choices so that their PCs can retrace their steps without getting lost.
(2), on the other hand, is completely standard for a wide range of play approaches. Classic D&D dungeons used to have "freeze frame" rooms - when the PCs arrive at the room the nymph is playing on her flute, or the dragon is asleep on its ledge, or the cultists are in the process of sacrificing the prisoners, or whatever. In the typical classic "freeze frame", the PCs have no causal connection to the events until they get there, though their may be an emotional connection (and likewise by the players): eg in another room the PCs may have acquired rumours of the prisoners, or learned that there is a musical nymph or a dragon in the dungeon.
Adventure paths have "freeze frame" rooms or situations. In I6 Pharoah, for instance (somewhere on the border between a classic dungeon and an AP), the dervishes will be looking for their missing members
whenever the PCs arrive at the pyramid. I assume that Dragonlance, and the modern APs, are full of this sort of thing.
I'm not running classic dungeoneering, nor APs, but would use a set-up like (2). For the reasons I've set out at length upthread, mostly in conversation with [MENTION=6775031]Saelorn[/MENTION], I think it would completely deprotagonise the players, who are searching for the prisoners, to have the prisoners killed off-screen. And if I wanted to make
time a factor, there are plenty of ways of incorporating that (eg via skill challenge) which would mean that the deaths didn't happen in a way disconnected from the players' knowing participation in the process of action resolution.