Creative Writing help?!

I have to write a four page long creative writing piece...and I have an idea but I don't really know how to develop it.

So basically, I'm doing it based on horror and my idea is so write like a diary entry from the perspective of the 'killer'. The last lines are going to be as follows:

'People always look for monsters. They look left. They look right. They look under the bed. They even look in the wardrobe. But, I’ll tell you one thing. They never. Ever. EVER. Look up. I dare you’ or something like 'Please put back your mirrors, we miss watching you'.

I need some kind of storyline because I don't know how I'm going to fill four pages with this idea! I need the detail and any good twists that can get me some marks.

Thank you for any help. English language is so hard for me!