This is a possibility but remember the third raikage did die of exhaustion after taking on 10,000 fighters. Maybe the 1st hokage died from chakra overuse.Yeah i agree.... The man who couldn't be beaten by Eternal Mangekyo Sharingan, can't possibly be defeated by anyother foe..... He was the top ninja of his time and invented a new elemental style, perhaps thats the reason he died... Remember the illness of itachi who was a prodigy in K.K genkai and also Kimimaro, he was also a prodigy in K.K genkai, so what i am saying is that perhaps Shodaime Hokage also got an illness like itachi and kimimaro got, and died of it....
No, Kakuzu was sent on a mission to assassinate Hashirama but he failed.Didnt kakuzu say he killd the 1st?