I don't buy Darth Vader's zero-to-evil-in-five-seconds shtick.
A few minutes before the whole "slaughtering younglings" scene, Anakin Skywalker had turned Chancellor Palpatine over to the Jedi. He recognized Palpatine as evil and an enemy.
(I just want to add that I hate the use of younglings instead of children - as if that makes this galaxy any cooler/farther away. They're freaking children, George Lucas. You don't need a new word for them.)
Anyway. Anakin had questions, and he was confused, but he still had enough sense to recognize evil and right and wrong. Then suddenly, he makes his decision.
I have no problem with that. What I have a problem with is that when Palpatine says, "Time to go slaughter some kids so you can learn how to save your hunny-bunny," Anakin just says, "Okay."
Even as a newly initiated Sith Lord - who earlier that same day turned in his friend for being an evil beeyotch - you'd think he would have at least some hesitation. Something like, "Hm. Are you... are you sure that will help?" Especially someone who's about to be a father, who later convinces this same evil guy that his son would be better off evil than dead.
No questions at all, Anakin? Really?
Later on, Darth Vader proves that there is still good in him - has been all along - but when it comes to slaughtering kids, he shows no trace of that. That kind of evil is not something that crops up in the middle of the night. Not even people in the Star Wars universe jump out of bed and say, "Hey, it's a great day to kill some kids in order to protect my lady love." There's some kind of build-up to that - at the very least, a strong history of mental health disorders.
Yes, there's the whole bit with the Sand People. But that had a driving motivation (revenge for his mother) and he likely slaughtered everyone because of a killing rage that he was already in, which blinded him to what he was doing.
He has no such excuse for killing the young Jedi. Even if you were to argue that his love of Padme drove him to it, Anakin still would have had a few questions, and I don't think it's a far stretch to say that revenge is a more believable motivation for killing than love. (If you ignore all religiously motivated wars, which I am at the moment. This is a different galaxy, after all.)
The more likely explanation for all of this is bad writing. And when it comes to the prequel trilogy, that's the explanation for a lot of things.