Andrea, Welcome to the Writing SE Site.
I wrote this as a comment, but I wanted to add more than a comment would hold.
To your question, yes, if you want us to feel anything about the girl's murder, you need to give us reasons to care about her. The question for you is, what do you want us to feel?
We could feel anything you want. Do you want this to proceed as expected, namely "poor innocent girl with limitless potential is mourned by all as sociopathic killer casually turns off her life?" Do you want this to be unexpected?
How was her relationship with the father?
The relationship between the father and the girl may have had problems. Was he abusive, manipulative, resentful of the girl? Does he love her, but not for her reality, but for the unachieved potential he imagines she should have had? What happened to her mother?
Is the father responsible, or does the daughter blame him? Did the daughter ask the killer to kill her father, perhaps directly, or perhaps by spinner her heartache to a gullible internet companion?
How is her ralationship with the killer?
Play out her relationship with the killer. Maybe slip towards Stockholm Syndrome, then away. Depending on the age of the girl, you may need to add some sexual confusion. Maybe the Dad was inappropriate and the killer respectful and chaste. You can make the conflict, the revelation, and the girl's journey as rich as you care to.
Why does the killer kill her?
Do we see her murder, or imagine it? Maybe she kills the killer instead. How does the killer know the father, and her?
It is your story!
I don't know where you want to take your story, but there is endless room for many stories within the framework you created. Every revelation can have a counter revelation or another onionskin of nuance.
Make us care, and feel, and guess, and hope. Make me cry.