If the creature you chose has 150 hit points or fewer, it dies. If the creature has 150-250 hit points it is stunned until the end of the creature's next turn.
As a way to make this spell more competitive from a player's POV, this has some merit.
But again, no such effort can be considered complete without offering a designer's comment on how this affects mid- to high-level gameplay in any campaign where the DM gives access to the spell to NPC/monster casters.
In short: these changes mean that the PCs will
never "outgrow" the effect.
If you're fine with that, i.e. your fighter hero still being defenseless against the spell even at level 20, then okay. But I would like to ask every tweak designer to state this explicitly so we know they haven't just forgotten about the other side of the coin - when the spell is used against the party.
Why am I obsessing over this? Because I'm not actually sure the spell needs improving. I'm actually fine with PWK being essentially an evil-NPC type of spell, especially considering there is no cheating going on (favoring evil NPCs over goody PCs) - just exploiting the good old hit point discrepancy: the fact that each hit point of "hero" packs much more punch than a hit point of "monster"!
Edit: riffing off your invention, maybe all is needed is to ensure the spell isn't wasted by stupid minutae such as the victim having 101 hp:
If the creature is reduced to 100 hit points or fewer before the end of its next turn, it dies.
This ensures the spell won't go to waste. If it doesn't die instantly, you and your friends have a whole round to shave off those niggling points that you miscalculated. Then the spell accomplishes its mission - even if it is the druid that plinks in 7 damage, that is still satisfactory if the BBEG had 106 hp when you cast the spell.
If the creature is reduced to 150 hit points or fewer before the end of its next turn, it dies.
I see what you're trying to do, but from a 5E POV this is cludgey. Feedback: I would phrase this differently, combining the two ways of dying into one effect. Perhaps something like:
Your word of power sets up a vibration within the target, lasting until the end of its next turn. If the target is currently having, or is reduced to, 150 hit points or below at any moment during the duration, it is instantly slain.