He was known as "The Demon's Asassin" because he works for a company who was associated with working with the Demons. People say that they needed a human soul to unlock the gates of hell. And of course it needed a human soul with Holy blood, which was you. Yeonjun was supposed to kidnap you and sacrifice you but he had to gain your trust first. He ended up falling in love with you and would quit running the association for you.