@Milanin
I think you misunderstand what happened.
This isn't a foreign force invading.
This isn't a fight for survival.
This isn't about bloodshed.
This is about how the guild and demons have already won and consolidated their power. Any adventurer is effectively a slave in the system, and the the "monsters" are off even worse. The reason killing one or two guildmasters is pointless is because society has already accepted and relies upon the existence of adventurers.
Kind of like how GATE: JSDF and Outbreak Company are two sides of the same coin (The sole difference is whether the initial diplomatic talks were successful), this is a scenario hinged on the truth of the Demon Lord being revealed. Demon King Castle: Black Mage is where the truth got revealed and wealth was shared, and this world is where the truth was hidden and adventurers and demihumans are exploited by the guildmasters and demons.
What you are calling the "There is another" was not done to prevent a war; It was done to quash any chance of ending the fighting.
It is when the adventurer's guild changed from a necessary service to a parasitic one.