Happened in one of our games too. It helped that the big reveal was we had been working for him the entire campaign already. Also it helped that the big bad was our king and not the evil advisor like we’d ‘figured out’. But he made some really good points, the gods were fake, they had abandoned us, we could execute the evil scientist on the spot. The only only cost was one city had to be destroyed in terrible torment, but then we could all ascend to godhood. But we would be gods, we could just cure them with our god powers after. The party was almost entirely convinced.

Except the murder hobo rogue who often didn’t pay attention to things, had attempted to kill a small child sick with a disease because we hadn’t found a cure(and failed though he stabbed her several times), and would spend his free time murdering people in alleyways during character down time(but only if he thought they committed a crime so it’s definitely not evil).

Our pet npc also didn’t want to so we were trying to convince him when the rogue suddenly tried to stab the king who, we already knew, could see into the future. The king did not get stabbed and the king’s bodyguard put the rogue in death saves with one attack. We were going to leave him there. But then the bodyguard turned to attack our npc for not joining and that was enough for us to commit regicide.