That's why he shouldn't just become selfish and power-hungry out of thin air.
He should become selfish because he feels he is being denied things (at first things he actually needs and we can understand).
He should become power hungry because he has periods of being powerless to prevent the sort of things we would like to prevent or to accomplish the sort of things we can understand at first.
He should start down the dark path not teleport to Darksville Darkonia.
Also if we aren't meant to know this is Vader until Episode V some of his really nasty stuff should be moved onto the Vader persona and moved away from the Anakin persona.
It would be way too obvious that Vader and Anakin are the same guy if Anakin is being all Vaderific.
I'm friends with people I've known when I was a boy.
The boy Anakin isn't the problem (the yipees aside) it's the jerk teen Anakin that's impossible to like.