Murder motives for a story?

I am writing a short story about 9 teens and 1 kid, (kid isn't really important, just added her so she can be a little sister to one of the teens.) at a house party, no adults. I want the story to be where each person is killed off by a murderer. But unfortunately, everything I have wrote so far has been to... show more I am writing a short story about 9 teens and 1 kid, (kid isn't really important, just added her so she can be a little sister to one of the teens.) at a house party, no adults.

I want the story to be where each person is killed off by a murderer. But unfortunately, everything I have wrote so far has been to predictable/bad. Any ideas on how I should write the story, or reasons that the murderer is killing these people?
Update: Thank you for all your feedback, kind or not. I have to write this story for a school project. I have had more ideas for the story than just this one. This is not a story that requires perfect grammar, as I am not even in high school yet. I have already written most of my story, but like I said, it is far too... show more Thank you for all your feedback, kind or not. I have to write this story for a school project. I have had more ideas for the story than just this one. This is not a story that requires perfect grammar, as I am not even in high school yet. I have already written most of my story, but like I said, it is far too predictable. Anyways, I have already changed the whole story, I believe it is better now.
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