I'm sure it's not, but GL did a poor job in showing us that transition so that when we saw it on screen, it appeared as though he was simply just giving himself completely to the dark side and then going off and killing people without a second thought about it. He just stared killing younglings and dropped on his knees and praised Palpatine out of fear and cowardice. I believe you, but I just didn't see the major transition from good to evil. It just happened so fast on screen.
It was probably Hayden's fault, just as much. After the slaughter of the Tuskens he should have come off as more angry and less whiney.
Though, George is either responsible for that or responsible for not correcting it.