It refers to current

Voldemort could possibly be the strongest wizard in DnD, given how he can essentially cast this as a cantrip with "AVADA KEDAVRA"

Quote from PadreMontoya >>

Kill as in 0 HP and save, or kill as in "roll a new PC?"

 New pc. 😏

amazing spell

Gotta love using power word kill on the party

AKA Avada Kedavra

...NOW!!!

you can use it on a creature (or player) whos hit points have dropped below 100.

Voldemort isn't the only wizard who can cast Avada Kedavra, but he's one of the few who won't hesitate to use it. SPOILER: Snape used it it on Dumbledore.

kills the bear, not the druid, druid is back into its human form with whatyever hp it had when it transformed

Polymorph and the poly morph is dead.  not 0 hp, just dead.  Tiamat to chicken to power word kill, tiamat chicken is just dead.

If your fighting something that knows power word kill, you probably have a way to resurrect dead party members. but they are dead, no saving throws. Revivify still works, though.

Power Word Kill - from their songs, Gelatinous Cube and When the Warlock dies- looks like these guys are one of us. 

https://youtu.be/pSOxJpKPg14?si=dY-JFiNfQTltN4Ic

What I'd say is if >100/100 deal 100 damage, if <100/100 then one hit

I think that a spell which deals 100 damage straight away is a bit broken but it could be really cool in homebrew. Power Word Killamanjaro!

Quote from KSly >>

kills the bear, not the druid, druid is back into its human form with whatyever hp it had when it transformed

 

 No, if you instantly die; like via Power Word Kill, you do not revert back with your remaining HP. You will revert back to your normal form, but you still die.

https://www.sageadvice.eu/if-a-druid-wildshapes-into-a-wolf-and-is-targeted-with-power-word-kill-dead-or-alive/

Quote from wagnarokkr >>

Quote from nerdsrus2019 >>

I had cast death ward on my paladin who then got hit by this spell.  Is he just dead or should he get back up with 1 hp?  

He's just dead.

The description of Death Ward says that it triggers "the first time the target would drop to 0 hit points as a result of taking damage". This spell doesn't do damage in the normal sense; the target is just dead.

 No, the second line of Death Ward protects explicitly against that:

"...If the spell is still in effect when the target is subjected to an effect that would kill it instantaneously without dealing damage, that effect is instead negated against the target, and the spells ends."

Power Word New Character