The quick turn of Anakin was bad in RoTS, but the fact that we never really see him suffer throughout the prequels makes it even less sympathetic. You wonder "why the heck would this kid turn so quick?" not only because Lucas did a poor (too quick) job of explaining it in Sith, but he fails to set it up in Eps1-2.
Take Episode1: We "hear" that he's a slave only by other characters talking about it (Padme says "I thought slavery was nonexistent...") but we never see anything BAD happen to him. He's a happy golucky whiz kid, who doesn't have a bad gig in that he works for Watto who, although shady, seems to treat him okay, and he gets to polish up on his building skills where he gets to build his own podracer from scratch! How many "rich" kids in Tatooine even had a podracer?
Sure, there's dirt on him and his mom's face, that could be a sign of a hard life, but hey, it's Tatooine, a desert planet! Everyone's dusty!
And from what it looks like, mom can put food on the table, and they have shelter, which is about average for Tatooine folks who aren't in Jabba's palace.
So really there was no attempt to establish a hard life (to the degree that Lucas should have to justify a "turn" later on). All we have is a kid who can build his own podracer, win a race which looked like he had fun, and then piloted a ship by happenstance which won the battle. Not such a bad life, eh?
Then Episode 2:
Apparently he's been in Jedi training for 10 years. Now I know it's grueling, but come on, it's a pretty good life in comparison. And what do we see? Petulance from the beginning. He's arrogant, whiny, oversteps his bounds, disrespects authority, and is in love with the most beautiful woman in the galaxy. Not such a bad life here either. He is on his way to becoming the most powerful Jedi, and the only thing we see of him suffering is the death of his mother, which he has a chance to work out by slaughtering the sand people (whereas a death unavenged can eat at the heart more than when you can actually take revenge on the killers). But even that turn is rather quick.
What I think should have happened
Starting w/ Ep1, the kid should have been a little older (maybe I just hated Lloyd's performance). Then we should see that he leads a hard life. Maybe Watto is tougher on him. They can't make ends meet, they get scraps of food from dumpsters, mom's beaten when she doesn't do the master's bidding to his satisfaction. In fact, show a few slaves leading bad lives (maybe a fellow child friend of his who is beaten).
Whatever it takes, we should see that he had a troubled and difficult life. Yet through it all, a strong will, resolve and defiance radiates from him. This hard life isn't taking him down, and despite it, he can and does succeed at things like building and winning the podrace.
He does want to free the slaves and ensure freedom throughout the galaxy because he wants to free people from any situation that resembles what he came from in Tatooine.
So at least now, when he's petulant in Ep2, we see it's because of the hard life he led, and how he wants to ascend up the ranks so that he can be part of the council and do good. Then when he returns to Tatooine, show that nothing has changed, it is still a slave-situation, and maybe have a scene where his dying mother asks why the Jedi and the Republic failed to help their planet? That would be the impetus as to why he wants to get more power to either be able to steer the Council to go to action, or to break out on his own since it's pretty clear that the Council is not a fast moving body of action.
Now that you have him feeling like the Council is part to blame, that democracy in action (often slow) is partially to blame, and that being "held down by OB1" is partially to blame, you can start to justify the turn.
And of course, in RoTS that turn scene needs to be elongated. Maybe have Palpy state "now that you've helped with Master Windu's demise you can NEVER go back!" and have Anakin fight against that feeling, saying that he can bring Palpatine in and make amends, and then have the Palpy work further verbal voodoo on him to drive home how futile his attempt at redemption would be, and how there is only one way to not only escape punishment by the Council, but to also restore order to the galaxy (and bring up his mom here, just as Vader used "sister" to set off Luke in RoTJ--"if you won't join me to save yourself...do it to avenge your mother!"). Palpy's most potent force has always been his silver (forked) tongue. Use it!
Again, this is a quick skeletal outline that I came up with in like 5 minutes, but I think something like this should have been followed as opposed to what we got.