Revenge is a powerful motive and can go both ways.
PC don't want to go after the cult, fine. Have the cult go after the players.
For example, in the last adventure, somebody the PC killed was a relative/loved one/special friend/etc. to the cult boss. Now that boss is bent on (not only the Tiamat thing) but on getting revenge on the PCs.
Can you "slow down" the timeline of the module and allow the PCs to do some short side quests? During these side quests the cult keeps intervening against the PCs. The PCs clear out the lost tomb of Bob, only to be attacked by the cult on their way back to town.
The cult just doesn't go after the PCs though, anybody associated with them is a target also. The inn they stayed at 3 nights ago is burned to the ground with a note left that states a similar fate awaits all who help, do business with, etc. the PCs. The PCs may easily be surviving the direct attacks, but these indirect ones will pressure them to do something. How long will merchants continue to deal with the PCs when anybody who does is themselves attacked? What will happen when inn keepers start turning them away because they fear their inn will be destroyed simply because the PCs stayed there?
The cult has declared war on the PCs. The PCs can either try to flee, die, or take the war to the cult (which leads back to the adventure).
