The Furies were summoned to bring about a loss to their victim. They were aiming to make that loss Grace's life, but when Freddie took the kill in her place, that loss for Grace was enough for the trio to feel they had committed their duty and stand down. It's not about getting their fill of blood as no random stranger could take the sacrifice for Grace. It had to be someone truly close to Grace, so she'd feel the impact of the loss.
We're told about some of the Idol's backstory in history, including that the last memories they all have of Zeus is him telling everyone to prepare for war. After that, everyone experiences amnesia and their memories black out until Athena comes to find them all many centuries later. In Greek Mythology, the Trojan War was manipulated by Zeus to reduce the human population, especially of the demigod population that he had a part in creating.
Since the games are well researched and take kernels of truth from real life Greek myth, it seems likely that Zeus manipulated the Trojan War into being for some motive and whatever happened during the real events of it forced a memory wipe of everyone involved. And as a result of the Trojan War, either Zeus was killed during these events, or perhaps he felt regret, shame, etc. over his actions and left to exile himself.
