Interesting concept, but here's the thought: Just what kind of Demon Lord VS Hero that the author was playing? In the old stories and games might be true. But, it's 2019! The Demon Lord didn't kill lv 1 hero not because they're relaxing. It was because they didn't know who the hero was until it was too late. Why? Because he was busy conquering things and being faced by excellent troops and fighters. Then when the hero slayed one of his generals, he just can't order his other generals to give reinforcements. This remain true in Myth, Canon, Legends, etc. Besides, in most common Stories, someone can be called "Hero" after they defeat "Significant Evil".
So well, another crappy isecrap.