It just felt rushed in the actual movie itself. like one second he's mad that the jedi's passed him over for a promotion then the next he's klling kids.
He killed kids in AotC, but because they're Tusken Raiders a lot of people give it the
TPM established that Anakin was idealistic with a slave mentality who easily got attached to people
AotC showed he believed in a dictatorship, felt powerful people should be in charge (and he was/should be the most powerful one, was willing to break the rules and threw his morals out the window when he felt he was wronged.
Once he helped kill Mace he really got rid of his last moral barrier, which was trying to be a Jedi, and just went all out with it.