He originally wanted it for himself. After finding out Ming Yi was poisoned, he wanted to get it for her instead.…
They should have come up with a better plot... but I also have to attribute the role not convincing because of the actor. This actor doesn't play villain roles well... The writers/director probably should have shown some of his obsessiveness in earlier episodes to provide more insight into his obsession for MY/MX in later episodes. It's more than just gratitude, it's admiration, it's idolization and then attraction... He thought MX is a male, but then when he discovered she was a female, and a beautiful one, he probably got attracted to her and wanted to help her in anyway, that lead to wanting her affection, and then wanting her.... there was a progression in his feelings towards MX/MY that wasn't shown, but would have helped in explaining....
Question for Chat: So I rewatched the fight scene between JBZ and MX in episode 1... Did you see MX using her fan "Embers?" The one that JBZ took with the intent of returning to MX one day.