Hashirama was given 2 choices. Whether to kill yourself or kill your brother then I will trust your clan. What if the only choice was for Hashirama to kill Tobirama. Would Hashirama go through with it to achieve his dream? Or would he spare his brother? Then the fighting would continue.
I think Hashirama wouldn't kill his brother. But it would be quite a twist for hashi to stab tobi in the stomach with a kunai.