Goodman Games

So.....if the cultist thinks they're being overrun, he makes for the storeroom, and sets off the bombs, leaving a sizable crater in the ground, and absolutely certain death for every person within 300 feet of the cottage (effectively 30d6 damage - 15 force, 15 fire). As characters are expected to be somewhere from eve 2-4 at this point there's just no way they will survive.

I mean playing it straight, if the cultists see the inevitable, its really just down to luck if they make a break for it and survive getting past the PCs. And if they do, everyone's going to die. Just seems like a very clumsy TPK written into the adventure. Of course I can modify, play it differently, and do whatever's right for the table. I'm just wondering why the heck it's written this way in the first place.

Old School fun is full of stuff like that.
Players cna make new characters. Character death and TPK is part of D&D.
I hate it when I'm sent to jail in monopoly, but it's part of the game.

What if when he throws open the doors, there's a chance for the characters to see the bombs the cultist intends to set off...?

Then there's action in trying to stop that attempt, or to run away, a mix of the two, and even other clever actions from the players...