I say that Anakin makes a CALCULATED decision to CHOOSE HIMSELF AND HIS WIFE, not joining Palpatine. The crux of it is that he feels that he must do Palpatine's bidding to get what he wants (Yoda's words don't offer anything. At least with Palpatine, he offers something that Anakin can grasp a hold off. Essentially, Yoda asked Anakin to give up his wife. And we all know he's not going to do that. He wants his cake and to eat it too.) But, that doesn't change the fact that he's also decided to get rid of Palpatine who he feels he has what he has. What Anakin wasn't counting on was the Dark Side dominating him.
Now, you are correct in that Episode II and Episode VI do a great job of showing that. But, I like the decision by Lucas of Anakin choosing to go this route, eventhough he knows what it might mean, because of love and fear of losing his wife.
I'm pretty positive that 100% of the diehard Original Trilogy fans would've never gone this way but I feel it makes for a compelling story and I'm glad he did do that. It would've been a very easy story for Lucas to show that Anakin gets upset at the Jedi and just decides to betray his friends. It would've been easy to show that Anakin was always a hothead and never was really a good person underneath.
The problem is that everyone knows he's going to the Dark Side. It's not a secret. So, in Lucas' mind, he has to figure out a way to make Anakin's trip to the Dark Side not what anyone expected. This is why he makes the crucial decision of showing Anakin at nine in Episode I. The threads of his fall are in that film.
I mean, I'll admit that Lucas could've made Anakin at least 16 in the first film and still have him live with his Mother on Tatoonie. The problem is that, in general, when everyone gets to that age, we all want to get away from our parents one way or another. He's not really dependent on his Mother at 16.
He still could've been good, selfless, and an earnest young man. But, leaving the home at age 16 isn't as tramatic as leaving at 9.