Okay, here is my spoiler free prophecy since so far I guessed everything right. Yohan is mentally unstable due to years of abuse, and something happened to his mother when he was found almost dead. As it was easily predictable, granting great powers to a mentally unstable child due to a mentally impaired (hyan) child tantrum was an awful idea. the impaired will not believe "the bad stuff about oppa" and get lured into a trap and be used as a hostage. Then, to win Yohan will somehow steal the powers of hayan parents and cause the vampire onslaught.
As for why he wants hayan heart i have no clue. Why did her parents say that they were "half blood vampires" like yohan? maybe he needs the heart of a pureblood (born from 2 vamps) to get the full powers, idk.