I Reincarnated As The Crazed Heir

I Reincarnated As The Crazed Heir is a Manga/Manhwa/Manhua in (English/Raw) language, Action series is written by Updating This Comic is About
Yang Jagak, who was assassinated through poison during the political wars of the
Murim. While he thought he was dead, he realized he was in the body of another person.
He found out that he is in the Demon Cult, the art he knows is the Heavenly Demon God Arts, and the owner of the body was the crazed heir who lusted after women.
Why did it need to be the Demon Cult, and why did it need to be this person?