Even more than most, this version of Sudoku begs for a feature that good online Sudoku games have, and this online Sudoku game doesn't have: the ability to write numbers two different ways so that you can tell the difference between a guess and a decision. Ideally, the ability to write multiple guesses in the same square, which some online Sudoku games let you do. Without any such feature, this game truly deserves the name Killer, and if that's what you're looking for, more power to you.