We all know how Tobirama died. He and the Second Raikage were ambushed by Kinkaku and Ginkaku. (correct me if I'm wrong)
Now if his brother Hashirama is so strong to have crippled Madara Uchiha, then who exactly killed the Shodaime later on? All it says is that Mito Uzumaki was left a widow because The First Hokage was killed during wartime. Was it a group that killed him or an individual? Really? Someone stronger than Shodaime and Madara? Is that even possible? (Ya know, besides the Sage of Six Paths)