So their goal was to destroy the purebloods. Which is the same thing he's attempting to do now.
But giving the hunters the power to damage purebloods isn't the same as giving them the ability to kill them. Even for a hunter, the ability to kill a pureblood is a difficult thing to manage. Only the most powerful can achieve it.
That's what he wanted me for when Rido returned. He couldn't do it. That's why he allowed our situation without putting a stop to it, the more I drank from you--the stronger I would be. He gave me his own blood. Ichiru sacrificed himself for it...
[Zero sits in thought for several moments, trying to work things out in his head a little as he sifts through this new information with the information he already has.]
You said he wiped his memories, so when did they come back? Why was he willing to go against the purebloods back then? Just to protect the humans?