Power Word Kill -- Max HP or current HP?

Question

Power Word Kill:

You utter a word of power that can compel one creature you can see within range to die instantly. If the creature you choose has 100 hit points or fewer, it dies. Otherwise, the spell has no effect (101 hp or more).

(copied from Functionally, how does Power Word Kill work?)

Does the phrase "hit points" mean current or maximum HP?

For example, if I fight a Tarrasque with 686 HP and do 587 damage, can I then use PWK to kill it?

Answer 1

It means current hit points.

When the phrase "hit points" is used without the word "current" or "maximum" associated with it, it means "current hit points." This is clarified by the rules in the Damage and Healing section of the Player's Handbook (p. 196, emphasis mine):

A creature's current hit points (usually just called hit points) can be any number from the creature's hit point maximum down to 0. This number changes frequently as a creature takes damage or receives healing.

So, when an effect applies to the maximum, it will always say so. If it doesn't say "maximum," it's talking about current hit points, even if the word "current" isn't actually there. (Also, as a sidenote: the PHB never uses the phrase "maximum hit points." It always uses "hit point maximum.")

Answer 2

The spell's text does not say "Maximum" therefore it is implicit to mean current Hp. As @Bloodcinder found as well PHB p196 has clarifying verbiage.

You utter a word of power that can compel one creature you can see within range to die instantly. If the creature you choose has 100 hit points or fewer, it dies. Otherwise, the spell has no effect (101 hp or more).

So yes barring any other traits from a creature to the contrary once you reduce it to or under 100hp (which a player usually doesn't know) you can kill it outright.

This also works on polymorphed and wildshaped creatures that have a lower hit point set due to the ability or spell. Since the specific verbiage on Power Word: Kill simply kills you (assuming you are within the hp limitation of the spell) when you when cast you never get to zero hit points, you do return to your normal form but you are a corpse.

Woe to the level 20 Druid flying around as a wren that comes across a paranoid Wizard that simply feels like targeting it with Power Word: Kill.