Hashirama Senju wasn't assassinated by anyone in a straightforward way, which is something that gets misunderstood a lot. The Naruto lore shows he died at some point after the founding of Konoha, but his actual death is handled off-screen. There's no big manga panel or anime episode showing a final battle. Some theories floated around forever, like maybe Madara came back and finished him, or it was some political betrayal, but those are all fan speculation without real backing from Kishimoto's work.
Honestly, I think the ambiguity is intentional. His death isn't the point; his legacy is. He establishes the village system, makes peace (however fragile), and then passes on. His death being from natural causes, or maybe from wounds accumulated over a lifetime of fighting, fits the 'Will of Fire' theme better than some dramatic murder. Having him be killed by a villain would have shifted the entire emotional weight of his character from being a foundational, almost mythical figure to just another casualty in the conflict cycle.
If you're looking for a direct killer, you won't find one. The databooks and supplementary materials just state he died during the First Shinobi World War, which implies he fell in battle, but the specifics are left open. It's a bit like wondering how exactly the First Hokage's era ended—we know the result, but the exact moment isn't dramatized. That lack of a clear villain for his death actually makes the world feel bigger, like history has gaps we don't get to see.