Here's a well used writing prompt. I got it from Brian Turner’s summer 2014 workshop at The Old Oak Dojo.
Write a letter to an enemy.
In 500 words or less express your contempt, reasoning or disillusionment toward your worst enemy. It could be an abstract member or the leader of an opposing military force like ISIS, Assad, or the Viet Cong. Or it could be a direct letter to just any Shitbag you've known. Set a stopwatch for 10 minutes. Write free verse or prose. Go.