... "The Sudoku Strangler"
If you are short of a plot idea, may I suggest an embittered crossword setter, or the setter of the Bridge or 'white to mate in three' puzzles that appear in papers, roaming the country strangling Sudoku setters in a fit of pique? Or someone, loosely based on
mr_snips, wiping them out to prevent the tragic waste of time and effort by all the people solving them. 