The curse had stopped them from loving. It had made them cut off, withdrawn, self-conscious and controlling over their feelings. It had trapped them, trapped the young ones in the fear that they would grow up without love like the older ones, and the older ones in the pain of watching the younger ones become without love like them. In truth, all the turning-into-a-different-Zodiac-animal-when-you-are-hugged-by-a-member-of-the-opposite-gender was just a side effect of the real curse. The real curse of the Sohma family was loneliness.
Then came the girl that might change all that.
She was impossible not to like. She tried so hard, and was so happy when others were happy, that everyone loved her. Everyone. Except Akito. And everybody and Akito knew that the curse would not be broken until the head of the Sohma family found love.
But then Akito would lose power over his precious family kingdom. And so he refused to love her.
He refused to love her by hating her. It was the only way. He couldn't just let her be, ignore her, he had to hate her or she might slip into his heart.
So he hated her best he could. But somehow, he could never bring himself to say it out loud.
He hated her. He watched closely over the young ones, just in case they let themselves get attached to this horrible girl. And he warned the adults, though he was quite sure they didn't believe him, that she was a wicked spirit who seemed nice but would lure them into a trap and make them dance like puppets. Like the Sirens from ancient mythology. So that's what he started calling her. The Siren. Although he knew, in his heart, that Sirens could prove to be irresistible.
He tried to bring himself to order the Dragon to erase her memories. Erase all traces of the Zodiac; erase all fragments of the curse. It would be better, his reign would last longer if she were gone, if the threat was gone. But he knew he could never do that. There was still that one part of him, that one part that wanted to be saved…
And so, Akito let her keep her memories. He let her make friends among the Sohmas, and maybe more than friends. He let her live, as he lurked inside. Always inside, smoldering like a smoke bomb, merely hating her.
That merely was what defeated him. That merely is what saved him.