He would say: "They must all be killed."
And to the guild and other fighters: "If you know the disaster would come why have you not layed out traps everywhere around the villages?"
@jonsmth
You clearly never read Goblin Slayer.
For Goblin Slayer preparing traps includes turning the whole plains and forrests around the village into a minefield of traps and obstacles against goblins if he knows that a flood of goblins comes.
So in his eyes they did not prepare at all.