By RAW, the Druid dies.
Wild Shape protects you from a few specific things:
When you revert to your normal form, you revert to the number of hit points you had before you transformed (and then take overflow damage).
If you revert and have hitpoints remaining in caster form, you don't fall unconscious as a result of being reduced to 0 hit points.
Power Word Kill bypasses both of these restrictions by killing the Druid rather than dealing damage or reducing its hit points to zero. Returning the Druid to life (or preventing its death) is not one of the things that Wild Shape does.
This is basically the same argument as the one for massive damage, except that there's definitely no sequencing of events. The Druid dies, and Wild Shape doesn't protect you from dying. Game over.
House ruling options are slim here. Killing a druid for using a utility form is harsh. On the other hand, relegating a 9th level spell to "knock a druid out of wildshape once" is also pretty rough.
Does the druids combined HP matter or just the wild-shapes HP?
Just the Wild Shape's. Wild Shape says:
When you transform, you assume the beast's hit point and Hit Dice.
Like most of your other stats, your old hit point total "goes away" while you're in beast form. Your hit points are the beast's hit points.
With that said, adding beast hit points to caster hit points, or using the Druid's base hit points for spells such as Power Word Kill may be a useful house rule.
This ruling is confirmed in the Sage Advice Compendium, the source of official rulings from WotC:
What happens if I’m polymorphed or Wild Shaped into a creature with fewer than 100 hit points and then I’m targeted by power word kill?
You die.
Jeremy Crawford
This has been confirmed by Jeremy Crawford:
If a druid wildshapes into a wolf and is then targeted with power word kill does the druid revert dead or alive?
If you have 100 hp or fewer, power word kill causes you to die. Notice that it doesn't say you drop to 0 hp.
So what is PHB pg. 66 "You automatically revert if.....drop to 0hp, *or Die* ." telling us? Form dies, Druid reverts, yes?
Beast form ends if the druid dies; things like power word kill can end you without reducing hit points.
Spells do exactly what they say, whether cast on PCs or other monsters.
Power Word Kill says a creature with 100 hit points or fewer dies. PCs are not exempt from this as they are also creatures and thus are perfectly valid targets for the spell. Do not pass go, do not collect $200, do not make a saving throw vs the effect, do not drop to 0 HP and start making death saving throws. They just die like any other creature targeted by the spell.
Note that this is a 9th level spell so it's meant to be powerful.
Best Answer
There is no canonical answer for which word are the various Power Words. See this D&D 5e reddit post regarding some tables' choices. Yes, I know we're talking about Pathfinder, but the Power Words go back to PF1e's source material... even so far back as the first Greyhawk supplement in 1975. Greyhawk, if you haven't heard the name, is the setting used by Gygax in the original iterations of D&D when he was modifying it from Chainmail. I know of no source, from Greyhawk or more recent, for D&D or Pathfinder, that defines the Power Words.
Regardless of what the word is, it cannot simply be hidden in speech. All spellcasting, even if it is only verbal (or even no components via metamagic), is obvious in the Pathfinder rules barring the use of special abilities such as conceal spell.
From the FAQ: