Hashirama Senju was the head of the Senju clan. He led one of the strongest group which is deemed to be on par with Uchiha. Moreover, he confounded the Hidden Leaf Village alongside Madara Uchiha, his former frenemy.
Nevertheless, his death remains shrouded in mystery. Neither the manga nor the anime series Naruto addressed the details of his demise.
Here are some of the theories relating to the ultimate fate of Hashirama.
He overused his healing ability
This is one of the popular theories among fans as Hashirama was known to possess a special power of self-healing.
He had the ability to heal his wounds which he sustained during several battles. Moreover, he was able to regenerate because his cells could multiply.
However, this Jutsu came with a drawback since it would shorten his life span every time he used his ability to heal himself.
Hashirama was killed in a battle
Some of the fans believe that the strongest Shinobi died during a battle since during those times battles happened on a daily basis.
However, this might be farfetched since he was regarded to be one of the strongest ninjas.
Nevertheless, some suggest that might have been outnumbered just like the Third Raikage. While few think he was assassinated, it seems unlikely due to the failed attempt of Kakuzur in the past.
Additionally, others believe that he suffered a grave wound during his final battle with Madara which ended up costing him his life.
He died because of an illness
Another popular theory claims that the First Hokage lost his life following his battle with a mysterious illness.
Some fans are assuming that an unknown disease killed Hashirama from the inside. Moreover, his excessive use of his healing power did not help in getting better instead it made his health worse and eventually ended up taking his life.