It seems that most of the characters who have turned to the dark side have committed murders around the same time i. e. Count Dooku killed Jedi Master Sifo-Dyas around the time he became Darth Tyranus, Anakin killed Count Dooku (and was an accomplice in the killing of Mace Windu) right before he became Darth Vader, and Luke, had he turned to the dark side, would have done so by killing either Darth Vader or the Emperor.
If murder really is a necessary prerequisite for becoming a Sith, whom did Darth Maul and Darth Sidious kill? (I know that Sidious killed Darth Plagueis, but he was already a Sith apprentice by then, so that couldn't have been the murder that turned him to the dark side.).
Note: the idea for this question came from http://www.moviemistakes.com/entry129269