It's likely full of intense emotions and unexpected plot twists. The relationship between the killer and their love interest could be very complex and compelling.
Often, when a shoujo manga girl falls in love first, she'll go through a range of emotions. She might become extra self-conscious, try to get the guy's attention, and have all kinds of fantasies about their future together. It can lead to all sorts of cute and sometimes dramatic situations!
The plot is about the unexpected romance that blossoms between a yakuza member and someone unexpected. It's appealing because of the unique mix of action and romance.