Then, for the finale, have them reach the town a third time, but this time, have it be just a few minutes after the BBEG has already arrived. Be sure to have the party run past Fred's burning cropfields and the mangled body of Mayor Meyers, who has been bent in half. As the party reaches the BBEG, describe in detail how the BBEG snaps Bob's daughter in half with two massive hands as he forces a half-dead Bob to watch while restrained in the BBEG's other set of arms, then turn to the party and give a big, toothy, and truly happy grin with his small, beady eyes looking down on the party. If the party hasn't realized how powerful the BBEG is yet, have him throw the dead daughter at the party as a Ranged Weapon Attack and throw Bob in front, then casually breath burning ash on the party, enough to down half of the party. Then, believing that the party is dead, the BBEG will meander off to destroy more things.
Hopefully, by now, the players should thoroughly hate the BBEG, as he has defeated the party easily, graphically killed beloved NPCs, and given absolutely 0 fucks while doing it.
Does this seem enough like an irredeemable monster that gives the players "don't fuck with this guy" vibes upon hearing his name?