I would argue that instant-kill spells act as "Fulfill Death Requirements" functions. They don't act on HP. I think that this is justified in RAW.
From the spell description of Power Word Kill in 5E:
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.
You are compelling the troll to die. It is foregoing any possible regeneration.
It does come down to DM's discretion, but I would kill the troll.
I originally said this, after further research I no longer believe it comes down to DM's discretion, I think 100% RAW the troll dies.
This is supported by these answers:
Functionally, how does Power Word Kill work?
Power Word Kill works just like it says it does. The spell's description is complete and exact. If the target has less than 100 hit
points, they die. Specifically:
There is no saving throw.
The spell doesn't do damage. It just kills the target. Damage
resistance or immunity has no effect. Nor does any condition immunity.
There are no special targeting restrictions; the caster only needs to
be able to pronounce the verbal component, and see the target in range
(60 feet). The target doesn't need to see or hear the caster.
Since there is no save, magic resistance short of outright immunity to
spells doesn't help against this spell. Likewise, Legendary Resistance
doesn't help. A counterspell cast at 9th level, or with a successful
DC19 spellcasting ability check, will still stop it.
Usually a spellcaster doesn't know how many hit points a creature has.
So there's theoretically some risk of the spell "fizzling".
It's an enchantment, which means that if the caster has the Split
Enchantment class feature from the wizard School of Enchantment, they
can affect two targets instead of one.
Likewise, it's a spell with a single target so a sorcerer with the
Twinned Spell metamagic option can affect a second target by spending
nine sorcery points.
This is, of course, a 9th level spell, and is appropriately powerful;
only a character with 17 levels in Bard, Cleric, Sorcerer, Warlock or
Wizard can learn it. So in terms of balance it needs to be weighed
against other capabilities and threats at that level.
Does Power Word Kill instantly kill PCs?
Spells do exactly what they say, whether cast on PCs or other
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.
The consensus was that the spell does exactly what it says it does. Trolls are not specified to be immune to instant-death effects.
I would also argue that a spell like Divine Word would absolutely kill the troll instantly.
You utter a divine word, imbued with the power that shaped the world at the dawn of creation. Choose any number of creatures you can see within range. Each creature that can hear you must make a Charisma saving throw. On a failed save, a creature suffers an effect based on its current hit points:
50 hit points or fewer: deafened for 1 minute
40 hit points or fewer: deafened and blinded for 10 minutes
30 hit points or fewer: blinded, deafened, and stunned for 1 hour
20 hit points or fewer: killed instantly
Regardless of its current hit points, a celestial, an elemental, a fey, or a fiend that fails its save is forced back to its plane of origin (if it isn’t there already) and can’t return to your current plane for 24 hours by any means short of a wish spell.
These two bolded sentences should give you a sense of exactly how powerful a spell like Divine Word is meant to be. A troll on the other hand is a relatively low-powered enemy. It's regeneration can be stopped by something as simple as fire, or acid.
Now let's break down the Troll's Regeneration ability, and how it functions, logically. To start as a given, Troll's can have two main states: Alive, or Dead. From the Troll’s description:
Regeneration. The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.
So, we know that this regeneration ability only applies to live Trolls. A dead Troll can't regenerate because it's already dead.
So we can get this:
Troll = Alive
While Troll == Alive:
Regenerate() #At TurnStart HP = HP + 10
if HP == 0 and RegenerateError
Troll = Dead
What spells like Divine Word, or Power Word Kill do, is set the Troll = Alive at the top, to Troll = Dead, instantly. It bypasses the entire regeneration trait.
From The Man Himself
And finally, most likely as a result of this post, Jeremy Crawford has answered this question: https://twitter.com/JeremyECrawford/status/1047626065194012672
If you're affected by the power word kill spell, it doesn't reduce your hit points to 0. It kills you. It thereby bypasses features that rely on you having 0 hit points.
The disintegrate spell does reduce hit points, but if it reduces you to 0, you're dusted. #DnD