At the risk of sounding loopy.. I have to say I can see how this happened.

You have a young lesbian teen, who is living with her grandparents. Why? Probably because her parents threw her away, or have drug problems, and the girl was dumped with grandparents.

The girl, feeling abandoned and unloved is being raised by her grandparents. The girl finds love in another lesbian teen and has probably felt better then she has in years, possibly her whole life.

The grandparents are conservative, don't believe in Lesbians and restrict her from seeing the girl she loves. The grandparents, by rejecting their lesbian grandaughter, make her snap. Feeling the only relatives in this world who should love me don't, they don't accept me the way I am, and are stopping me from seeing someone I really love.

The girl she loves contacts her, showing how much she cares. The girl sees the only one in the world that truly cares for her is her new love. She's angry, upset, sad, etc. and takes out a lifetime of rejection and negative emotion by killing her grandparents. Gets 3 consecutive life sentences at age 15. Sad. Someone should have unconditionally been there for her.