Let's face it the greatest villain of all time has to be Darth Vader. While he ended up having a change of heart right before he died, he did absolutely dreadful things while he was still evil. To start, right after Anakin Skywalker crossed over to the dark side, he walked right into the Jedi temple with a squadron of clone troopers and murdered EVERYBODY there. He is seen on film killing of the the young Jedi children in training with no remorse. Next he goes and kills the leaders of old droid armies simply because he was told to. Following that he force choked the woman he loved just because she was with his former master trying to save him, this ultimately led to her death as well. This did upset him but not for very long. He attempted to kill his former master and then screamed how much he resented him after losing. All of that in one prequel movie! Not to mention the slaying of all of the sand people including the women and children after the death of his mother while he was still a "hero". To immediately start off his reign of terror with the Sith in A New Hope, he destroys an entire planet filled with innocent people just because the princess, his unknown daughter, refused to give him all of the information about the rebels. After this he continues destroying and terrorizing the galaxy, sending troops to kill his step-brother and his wife. Even after he watches the Death Star, with most of his soldiers and presumably some friends if he had any, get destroyed, he showed no emotion and just attacked more and more rebel bases searching for his son all while building another death star. When he finally catches his son he tries to kill him, giving him the options to either become evil or die. To his son people! Everything else was nothing but destruction until finally after watching the emperor slowly try to kill his son, he realized what love was again and became good long enough to kill the emperor and also receive fatal wounds in the process. Sorry that doesn't make up for the millions of dead innocents left behind in your reign of terror Vader. All of this adds up to Vader being the greatest villain. He's murderous, emotionless, and will do whatever it takes to achieve his goals which are always bad.