Why Are The Demon Lord And Hero Friends!?

The Demon Lords,
Known as the most malicious beings in existence. They live to destroy, to consume, to take over the land’s of the humans and purge them from the moment they come into existence. The 56 lords in their 56 dungeons plague the world of humans.

The Heroes,
Known as the embodiment of justice and righteousness. They live to protect, to save, to redeem the human’s the moment that they enter this tainted world. The 56 heroes are raised and die just to defeat their designated demon lords.

Both of these beings are not of this world, they are all chosen from the various wandering souls that plague the underworld. They are brought to bring balance to this world, with their memories of their previous lives wiped except for the most important parts.

So, as Damon and Artia arrive on opposite sides of the battlefield, with Damon as the 56th Demon Lord's replacement and Artia as the opposing hero, everything should be in balance.

…..At least, that’s how it’s supposed to be. How do you kill someone that was your best friend in your last life?