Many things happen in an initiate's inner world. This is my shot at what might turn a Jedi into a Sith.
Anakin, before his mother's body, wishes to kill them all. But his duty and his love and "how would Padme" stop the immediate action. So he finally crumbles and lets loose only a blood chilling cry that resounds in the Force and apparently vanishes.
"Think of absolutes, the dark side does", had said Master Yoda, or words to that effect. Was it the Council or was it himself? It didn't matter. She's dead. Shmi Skywalker's dead. Alone, in pain and dead.
One may pick things apart with a hydrospanner, or with the burning purity of a saber. But guilt seems total, final, it can't be picked apart. And drowning in this absolute, Anakin never noticed he felt no guilt over the wish to kill.
AN: I'm trying to keep Anakin IC according to his future Vader persona. It may seem OOC for some of the more recent movies. I want to write more but make no promises.