Invoked: At his trial, Mordecai plays the victim, faking tears and pleading his innocence. He does so not because he's actually innocent or because he thinks the jury will believe him, but to tug the heartstrings of any sufficiantly emotionally pilable potential fangirls who might be following the case. He then carefully sifts through any letters or e-mails he receives, looking for easy marks.
Exploited: Mordecai's victims are all MonsterFangirls, the more he kills the more he attracts, which he finds terribly convenient.
Deconstructed: Dahlia latches onto Mordecai because he makes her feel like she means something to him, providing a sense of love and support she hasn't found elsewhere. Her friends and family turning on her over her caring for such a monster only reinforces her belief that he's the only one who really cares. When he decides she isn't useful anymore and murders her, her death is considered acceptable and her own damn fault, receiving little sympathy.
Reconstructed: Dahlia had a supportive family and friends who stood by her even as they tried gently turning her away from Mordecai. She rejected their attempts to help escape his influence, and her death is treated as a tragedy simply because it feels preventable, with her loved ones wondering if there was anything else they could have done to save her...
Dahlia, a deluded Yandere, targets the officers who arrested Mordecai, reasoning that if they hadn't taken him to jail, she might've met him under better circumstances and been able to start a new life together.
With Dahlia's aid from outside, Mordecai is able to escape prison and go on the run.
Back to Monster Fangirl. Really, it's just a misunderstood trope, though... You don't understand it like I do!