She's doing what she's doing because she's desperate to save her father, but Jeffrey can't have any kind of empathy for her. The only empathy he has is for his precious parasite Wyatt. And even when she tries to be nice to him and emphasize with him he acts like a little bitch to her. She even tried to take up for him with Veronica who she's afraid of.

I think Melissa is going to try and kill Veronica. She's getting that fed up with her. But, of course, unlike Candace [another parasitic user who wouldn't lift a finger to save her parent's life] Jeffy won't have any sympathy or try to protect Melissa.