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If Vader says "I must, obey my Master." .. Why does he disobey Obi-wan in Episode II as Anakin? o.o

Maybe he respects Palpatine...?? I'm not sure.

He does, but he respected Obi-Wan, no?

dark side training sith stuff? i dunno

Because he's goint through those dark adolescent years of being a troubled and rebellious teen. There's also the fact that he's a guy and like any other guy his age, he got some uncontrollable hormones, especially whenever the matter is related to Padme.

Okay, I was just rambling on for the fun of it. I don't really know if what I said even applies.

in EpII Obi-Wan is his master, in the OT it's the emperor, that's not difficult

Dude..

He didn't obey Obi-Wan, but he did the Emperor.. I know they were both Masters, but why one and not the other?

again... in EpII he obeyed OB1, his master at that time
in the OT, he obeyed the emperor, his master at that time

Dude.. he promised Padme that he would find the killer, when Obi-Wan said no. -.-

maybe he bacame more loyal....dark side and such

Dark side was basically a major craving of power.....

that's cause he got feelings for her, that's the entire point!
but you fail to mention those ten years before EpII where he probably was a good padawan
if he really didn't listen, why would he go and save OB1? he's like a father to him so of course, from time to time he doesn't listen and most of the time he does, just as with the emperor! ("I told you to wait on the commandship!"

Here's the thing though: Vader says "You don't know the power of the dark side" before that. He's implying in that speech that the emperor has power over him. Basically, he's saying "I don't WANT to obey him, but I must." When he was Anakin, he had a choice.