That is the First Demon King’s opinion.
While it is true the demons are an aggressive race, and their way of life means their way to grow stronger is through surviving battles, they have individualism as much as other races. So if everyone would stop to think, there would be many different opinions about how they could have proceed.
As
pointed out in other reply, the demon race develop in place where resource are scarce and being aggressive is a matter of survival for their whole tribes.
But things had changes. They have expanded beyond their homeland and get a chance to develop culture.
Of course, how the war is settled would matter.
For example, if the pan-human alliance demand the demons to return all the land (practically pushing them back into their homeland), demand ridiculous amount of reparation (and forcing the demons into eternal debt servitude), or discriminate the demons as evil oppressors and war criminal for generations to come... Would be understandable if the demons snaps and starts to actually hates human back.