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
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.