Noah's slaughter wasn't random. Artamo had created a conflict of ideologies, and some of the potential members of Heaven and Earth's Cancer had been infected by his world in ways that Noah couldn't override.
The issue was simple. Some turtles, magical beasts, cultivators, and hybrids seated at the table had lost their desire to reach the peak through battles. They wanted peace, so they had become useless to Noah. There was even a chance that they could go against him in their current state.
The slaughter was a declaration that the other beings understood. Everything had reached the point of no return. The final battle was upon them, and they had to pick a side.
The reaction after Artamo's death told Noah what his potential underlings had chosen. Those that had remained confused about the sudden disappearance of the banquet could leave. Instead, those that had been angry enough to attack Noah had to die.