So, how long before the residents begin turning into the monsters?

The monsters we see come out of the forest and living in the caves are not the original monsters.

None are dressed in clothing from before the 1960's-1970's.

This seems to indicate that the monsters are generational in nature and thus, likely not the ones who made the bargain to begin with even though they can be brought back to life. >!(So was Roger...brought back to life, not animated)!<

None are dressed as a pilgrim or a civil war-era person >!(except for various incarnations of Jade)!<

This makes me think that the line, "It's happening again" may refer to both the slaughter AND the creation of these (newer? reincarnated?) creatures to replace the older ones. Some of these monsters may very well have been townspeople... that possibly knew Victor.

Could there be some mechanism that the MIY uses when he kills to later revive them as a monster/host for reincarnation?