No He probably died due to 2 things:
1. Sickness
2. Age( Dieing in his sleep or etc)
But there is no "he got killed".
He also, probably died after he spread the tailed beasts for peace conferences, and little bit before the first war began. Because Tobirama was the Hokage during the first War. While Hashirama was the "Hokage" (Strongest ninja) During a Era of War.