This comment made me re-think whether it was Tobirama or Hashirama who caused Izuna's death.
Also if we look closely at page 13, we clearly see that Tobirama slashed Izuna. However, Hashirama is standing close to Izuna, so is it possible that Hashirama was already fighting him and caused several deadly blows to Izuna, and then Tobirama used FTG to poped out and landing a killing slash. If this is true it would make sense that Madara really blamed Hashirama for the death of his brother.
Let me know what you think. I'm still leaning towards Tobirama causing Izunas death because he was his rival and this should be a display of finally settling their disputes.