Just humans being humans, as always. The Empire not only survived the war with demons on all side completely surrounded but thriving afterward, probably developed an isolationist and even xenophobic ideology, now if that same Empire intent on large scale conquest or feel their existence being threaten by outside force or simply want to monopolized magic, would made sense that they would want to eliminated the threat to their future. Or they could've made a deal with demons or already being infiltrated but that's no fun, human greed for power is more interesting