I am assuming that you're asking as you've watched the most recent, episode nine. The answer lies back in episode two at
In episode two it is mentioned that Kanon and Anos made a promise that if Kanon were resurrected that it would be as Anos' friend when they end the war between Humans and Demons. If Kanon were born human again as a hero he would be destined to fight the demon king again, where as if he were resurrected as a demon, this would not be the case. This would be why Lay was born with extraordinary swordsmanship abilities, as well as explain partially why his seven hearts were split to maintain in part his heroism in reincarnation.
Please note that at this stage in the anime this is still semi-speculation, but rather credible speculation at that. Anos confirms this at
11:00 - 11:14 in the anime in episode nine.