Tobirama and Izuna were the same. They both had hatred torwards each other's clans. Uchihas killed many Senjus and Senjus killed many Uchihas. Their big brothers created bond with each other but those two were influenced by their families so they both hated each other's clans. If Izuna lived longer then he would hate the Senju like Tobirama to the Uchihas.
It's like Hashirama let madara live as they were friends and Hashirama knew that Madara could turn evil at any moment. The same goes for Naruto ans Sasuke.
