Madara saw everything with that sharingan, and it was why he could react.

He was not certain why he reacted this way either... what even pushed him to do this.
He kicked his little brother's blade away, knowing the younger had little chance to stop himself, and then, then.. there was the matter of the Senju...

Tobirama was going to strike down the Uchiha. There was no hesitation as he plunged his blade... but then.. then it pierced something he did not foresee. How had he gotten here so fast?
Madara had shoved Itachi out of the way, blocking the strike that would have pierced through skin, inbetween ribs and reached his heart from the back.
Instead, the blade was there, having gone right through Madara's arm. The boy, sharingan filled with something almost demonic, had gripped the end of the blade with his free hand, effectively stopping the Senju from taking out his sword. All he could do now was leave his weapon if he wanted to run.

No Senju hurts his clan. None of them...