From the tone of your post, I assume that the point is that the character's death is unrelated to the conspiracy. Having him murdered by the conspirators, thus proving the truth of his suspicions, is well within the usual conventions of fiction.
Assuming the poor guy just gets run over or something, John Yorke's Into the Woods: How Stories Work and Why We Tell Them might help you. While discussing the random death of (spoiler below)
Omar Bradley in The Wire,
… [It] works precisely because it's so narratively wrong; it
undercuts the classic hero's journey by employing all its conventions
up to the point of sudden, tawdry and unexpected death. Effectively
saying this is a world where such codes don't operate …
That last phrase is the key to making this type of ending work. You have to replace the thing the reader thought he was going to experience (triumphant success, tragic failure) with something even more emotionally charged, usually the awful reminder that in real life real death frequently turns up without an appointment.
The thing to avoid is looking like you, the author, got a phone call offering more remunerative work and wrapped up the story in five minutes flat, not caring that you'd left the conspiracy plot line unresolved.
To do this, you could use the conventions of fiction even while planning to subvert the biggest one. Insert into your story foreshadowings of sudden death and great plans never to be completed. Robert Burn's poem To A Mouse or chapter 12, verses 16-21 of Luke's gospel might be studied for quotes or metaphors.