I know this will sound ridiculous at first, but hear me out before jumping to conclusions.
We're starting with a happy-go-lucky slave boy, whose mommy dies and he goes a little nuts. Then he gets married, which is a no-no. It's pretty safe to say his fall to darkness comes about because of his love for and marriage to Padme, and he sides with his friend Palpatine, who wants to kill all those he once held dear.
But is that alone enough to turn him into one of the biggest mass murderers in galactic history?
LucasArts and George Lucas seem to have more control over these enormous media projects than individual EU stories. The Clone Wars seems to be no exception.
On the one hand, we see the Jedi, often as not, commanding Clones from behind the scenes, seeming to take little heed of their humanity.
On the other hand, we have Anakin, always at the forefront of battle with the Clones. He even names the ARC Trooper he works most closely with (Alpha). To the boy who grew up outside the Temple, thinking that all creatures should help one another whenever possible, they are more than organic Battle Droids.
Will he at first justify killing his former comrades due to their seeming coldness to the Clones, thinking he is merely ridding the galaxy of worse killers, then slowly slip further into darkness until he eventually becomes the monster we see in ANH?