The customs of the Demon Continent are very different from that of the Fairy Human Continent or the Beast Continent.
There are many races that can't be distinguished from monsters, like Two-Legged Lizards, Four-Arms, and Centaurs.
The discrimination against the Demon Races (especially against the Demon Races with strange appearances) was deep-rooted at the time, and as a result the war escalated into a worldwide scale.
Though there are some lingering resentment, nowadays discrimination against the demon races is not so openly done, and relations between the continents had cooled down enough to do trade.