I think the path starts way back in episode I. First he is taken away his mother left in bondage. Then in front of the Jedi council...
YODA: Hmm... how feel you?
ANAKIN: Cold, sir.
YODA: Afraid are you?
ANAKIN: No, sir.
YODA: See through you, we can.
MACE WINDU: Be mindful of your feelings...
KI-ADI: Your thoughts dwell on your mother.
ANAKIN: I miss her.
YODA: Afraid to lose her... I think, hmm?
ANAKIN: (a little angry) What's that got to do with anything?
YODA: Everything. Fear is the path to the dark side... fear leads to anger... anger leads to hate.. hate leads to suffering... I sense much fear in you.
Even still in episode II Anakin's ego is getting the best of him. The death of his mother does not help. The massacre of the Sand People reminded me of the scene in Lawrence of Arabia where they massacred the retreating Turkish army 'Take no prisoners!!!'. Like Lawrence as much as Anakin abhorred killing he very much enjoyed the power to do it.