He was reluctant to do this, as he still wasn't sure who this voice belonged to. But the prospect of getting back at Gaius sparked a taste for retribution inside him that he couldn't ignore.
He was mad at the god for many reasons, the most recent being that he disintegrated Luna. If she hadn't been a soul manifestation, that would have been the end for her.
And the thought of losing her to a fickle god who couldn't manage his anger? That set him off.
But there were other reasons, too. Like the fact he was hunting for Psyche, or Nemus, as she was now called, and Nemus was his ally.
In addition, now that Gaius had seen how close his essence was to hers, there was no telling when he would decide that Astaroth was too much of a threat, and come back down here to put an end to him.
Or worse. Go on Earth and kill him.
Astaroth let the voice guide his actions, listening to her commands.