Ooh intriguing, I will put that on my list of things to do. I usually stay clear of such genres since I generally when reading such figure out who it is in the first chapter so it might be an interesting challenge for myself to try to make it harder for the reader.
Addition: you can write it in dual voices: one 'main' character, and the killer. As long as one's careful not to give too many details as the killer it's possible to hide who it is - even if it would be the main character However, there is also a suspension to know who the killer is as a reader, but understanding that the characters doesn't.
The subtle difference between sitting like "WTF IS IT???!!" and "OMG IT'S THAT ONE, HOW CAN YOU MISS IT?!?!?!" Kinda awesome, both of 'em ^^
Write a short blurb about someone's last moment in life before they die. Set up the scene, like a dark place or nighttime, set the weather, set the character and then as you get introduced to that character you realize that this is the moment they are dieing and it's the end of the story. I don't know, I get 'dark' or something.
You could do the opposite too, like predicting someone's future for their whole life as their being born.
But you have to push yourself to make it only a page long. CHALLENGE! Go for it!