For people saying "this is bullshit because before vamires were living fine with humans", you forgot that it's not the same kind of "vampire". Because of the experiments the blood mutated in Miyabi and became an overpowered virus which infects everyone into monsters, a way more extreme version of the old vampires, which can also mutate into demons.
So Akira is right, it's important to kill every single vampire to make this mutated vampire blood disappear for good. The chief is living for now because he's a big help. I do agree that his brother should have just joined their side, though.