Jungkook was a loyal servant of God, an Archangel who did what was right and moral and never failed Heaven. He had a list of duties and responsibilities, and carried a lot of power due to his title as Archangel—he was trusted. It was given to him, then, the mission of killing you, one of the highest ranked demons of Hell, a direct heir of Satan, and even if Jungkook knew it would be hard, he couldn't have predicted /how/ hard it'd be, nor how you were not just a demon, but a demon of lust.