Except that’s the point, they will not be having fun. Nor will you, nor will any of the other players. Because that setup is not fun. And presumably you’re hosting a game for your friends with the intention of everyone having fun, so it’s best if you find another tact.
There are ways. You could, for example, set up a bbeg where that’s his whole deal. The townsfolk are scared of this guy because he has the supernatural power to just kill you, straight-up. Maybe the questline leading up to their encounter involves the players finding defenses or counters or sabotaging his supply of spell components or whatever, such that, if they DO get power-word-killed, it’s because they had ample opportunities to not, and failed to take them.