And yet he was still a Jedi -- which is a title for someone in what is basically a religious organization -- up until the third film. And even then, I don't recall him ever saying he was becoming a Sith; it was just assumed when he became Darth Vader.And .... those were the moments when he was least like a Jedi ... and most like a Sith. Anakin's reliance on the Dark Side as a shortcut was a major reason for his turn to evil. Slaughtering the Tuskens, especially the children, was an act of evil. Just because he was a Jedi at the time doesn't exonerate him from the atrocity, or the evil of his actions. That, by the way, was an act of passion ... a rejection of peace ... and a conflict that helped increase his "power" and "strength". A very Sith-like (in other words, evil) thing to do. Why do you think Palpatine uses Anakin's conflict with Dooku at the beginning of RotS and Luke's conflict with Vader in RotJ as ways to increase the power and Dark Side influence in his targets (Anakin and Luke, respectively)?
Once again, and I realize this is really tough for you to grasp but I have faith in you: Jedi/Sith = titles. Codes = religious philosophies. Individual = varied.
Good Sith can exist just as easily as evil Jedi, just as easily as evil Catholic Priests or good LeVeyan Satanists.
Name some good Sith.