not 100% sure but... did someone tell Hashirama that Madara lived after their fight?
cause i only remember them telling Hashirama that Madara was brought back an Edo?
why would he state that he doesn't know how Madara came back to life... if he already knew it was by Edo Tensei?
don't have time to write much, i'm at work... *
however, hashirama says; "i don't know how madara could come back to life"
perhaps this mean that Hashirama did see Madara alive again? before Hashirama died...?
and maybe Madara was the one who killed him? but that only a "maybe" because it wouldn't make sense if Madara killed Hashirama and then stated afterwards that Hashirama is the only one who can stop him?
have a nice day