With the errata (no longer so recent), it is clear that true polymorph can be dispelled, even after it has been concentrated on for the full hour. On page 185 of the SRD, the spell description now says:
The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the transformation lasts until it is dispelled.
It doesn't appear that the "drops to 0 hp" counts as a form of "dispel", though. That would be the result of something like dispel magic. Instead, it looks like it's part of the duration rules that are changed when you concentrate for the duration: "lasts for the duration or until the target drops to 0 hit points or dies" changes to "lasts until it is dispelled".
Given that, it appears that it becomes possible to have a situation where someone living has been transformed into a monster, killed, and then not immediately transformed back. Under those circumstances, what would the result be if you dispelled the transformation on the dead body in question? Would it be the old body and back to life? Would it be the old body but dead?
(Interesting side question: what happens if the polymorphed-and-then-killed person receives a reincarnate effect before the dispel?)
I would also accept an answer that can prove (most likely by including additional rules information that I had not been aware of) that my earlier interpretation of the errataed rule was incorrect and that dropping to 0 HP or death will immediately dispel the polymorph effect.
Pertinent earlier question: When true polymorphed creature dies, does it revert to its original form?
Best Answer
The creature won't be raised from the dead
The spell description says:
Therefore, if you dispel True Polymorph from a creature, or if this creature reverts being at 0 hp, it gains hit points of its true form.
Lore-wise, killing involves departure of the creature's soul, and polymorphing does not:
Various resurrection spells explicitly say they bring the soul back:
If you dispel Polymorph from a corpse (which is an object, not a creature), the corpse is still a soulless corpse, since Dispel Magic is not a resurrection spell. Will it be reverted to its true form is up to the DM, but I suppose it will be — otherwise dispelling won't have any effect, but the rules say it have.