I think it was explained earlier that whenever the demon king dies, they get reincarnated and they sort of instinctively know that they are the demon king. When they grow to a certain age, the instinct becomes stronger and they must become the demon king no matter what. His right hand man said that even demons are aware of who the demon king is. It’s like a law of the world that the demon king must exist, the same way a force that will kill the demon king must also exist.
Sorry if this sounds a bit confusing ╥﹏╥
Well like VesperingViper explained, it’s just like how Phelan gets that uncontrollable urge to kill Yuma in the hero’s body, and Yuma originally had that inexplicable hatred for Phelan because of Sacher Blythe’s body. It’s simply the laws of this universe set by that bitchass goddess. My personal theory is that the last few cycles of the demon king and hero may have constantly been falling in love or questioning their purpose and the hero lost, so the goddess brought in an outsider to prevent her favorite (the hero) from losing again. Except she pulled in the most unhinged mf and he’s actively against her purpose.
there's part where phelan said he deceived yuma (to be demon king) for yuma's own good, also he said he only gonna reveal the reason to yuma after it's all done, to guarantee yuma will never leave him. so my guess the reason is something yuma will hate. also maybe with the vacant of demon king role it's gonna be bad in the future, since no one control the demons, in a way with phelan claiming the demon king position he can protect yuma.

Wow i am not understanding the soul thing anymore