I can imagine that Tobirama was a menace before the founding of Konoha, there's something inside him that broke so badly after his two brothers' deaths. He became so zealous and willing to strike down any Uchihas who tried to harm his anija and the rest of his family with no mercy. Every time he thought about his actions, the memory of Itama's death kept playing in his head, so he justified his actions as necessary..an 'eye for an eye' kind of thing. Hashirama always reminded Tobirama not to get carried away, which he listened to, but he drafted Edo Tensei in secret instead. (he's low-key a madman, but he is lucky to have people who reminded him of his humanity. That's why he looks chill asf now.)