Part of the Troll's Regeneration feature says:
The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.
Divine Word says:
[…] On a failed save, a creature suffers an effect based on its
current hit points.
- 20 hit points or fewer: killed instantly
And Power Word Kill says:
If the creature you chose has 100 hit points or fewer, it dies. Otherwise, the spell has no effect.
In addition to such spells, there are more general effects that cause death, mainly massive damage and failing three death saves (if the DM actually rolls death saves for the troll).
It seems like the Regeneration trait enumerates the only case where a Troll can "die". But is Regeneration specific enough that it beats all other death effects?
I am looking for a RAW interpretation. Were I running a game and someone Power Word Killed a troll, I would have it die – but I'm not convinced that outcome is RAW.