That scene always was just so jarring (as was the fall itself).
He certainly did a few questionable things in the Clone Wars (definately getting more jealous and occasionally dropping the odd force choke here or there) but his fall was just so sudden. Other than killing Dooku and some light genocide on Tatooine, there were not too many other acts, where you could say 'Yep, evil'.
And then suddenly, he's kneeling before the Emperor, and then off back to the place he just left on a speeder, to slaughter a ton of.... children?
Like I know he murdered a bunch of kids a while back on Tatooine (and told Padme about it, when they first started dating, and she was all like 'we all make mistakes sometimes Annie... like WTF?) but still.
Slaughtering the separatists first perhaps, and then Jedi, and then the kids.
Palpatine: Rise, Lord Vader, for your first task as a Sith, I want you to return to the Jedi temple where you were raised and trained, and where your friends all reside, and murder everyone inside. Do what must be done!
Vader: Even the younglings?
Palpatine: Especially the younglings.
Vader: Errr... that sounds a bit intense. Like, you cant get someone else to do that?
Palpatine: We didnt join you, you joined us remember? In for a penny, in for a pound my young apprentice. Now off you go. I have an important holonet call I've been dying to make for some time now...