Question

Ideas on a short horror story

Answer 1

The ghost in your closet. 

Answer 2

This is just off the top of my head but what if the main character has an inner conflict with his or herself? like multiple personality disorder, this other personality tells the main character to do things like killing those the character loves most. ( you could use some flat characters here like aren't really important but help push the story forward like a parent-sibling etc.) the main character knows this is wrong but this other personality (in the main characters mind) is more violent and has all the power and isn't above causing blackouts to take control.  anyway hope i helped please let me know if you need more help :)