The recent databook said Madara killed all the Senju. Must have been revenge against Hashirama in a sense.
Here's one; did Madara take Izuna's Sharingan as he was dying from the wound from Tobirama, or did Izuna willingly give him his Sharingan?
How did he kill all the Senju and let Tsunade survive? She's a direct descendent of Hashirama, that he must have known about since she was around during the 1st reign.
Just another huge oversight it seems.