The entire Star Wars series features the constant and ever-changing battle between the forces of good (the Jedi) and evil (the Sith). In Revenge of the Sith, this clash not only occurs between the two sides but within Anakin Skywalker himself. Yup: it gets personal.
His fight to resist his inner hatred and anger becomes one that will not only affect himself but will have reverberating ramifications across the Galactic Empire. And, because we all know what happens to Anakin at the end (cue "The Imperial March" theme), one of the major recurring themes is the power of evil to overcome good, no matter what the intentions.
Only a Sith deals in absolutes because they are evil.
There isn't a difference between good and evil, merely a difference in perspective.
At the very end of Return of the Jedi, Luke Skywalker is able to finally rescue his father, Darth Vader, and bring him back from the dark side just before his death—revealing that he did still have a little bit of good in him, buried deep down.
In Revenge of the Sith, we learn why Darth Vader turned to the dark side: fear. Specifically, it was the fear of losing his wife, Padmé. Then, George Lucas had to show why, after failing to save her, he would remain with the Sith: because all those who gain power are afraid to lose it.
A fear of losing Padmé was the only force strong enough to make Anakin switch to the dark side.
Anakin was headed toward the dark side no matter what; it was only a matter of time.
In order to make Anakin lose faith in his beloved Jedi, Chancellor Palpatine works to erode his trust in Obi-Wan and the Council, effectively isolating Anakin and paving the path for his conversion to the dark side.
At the same time, Anakin's closeness with the Chancellor leads to a healthy dose of distrust from the Council, making the situation insanely tense. Throw in some political intrigue and matters of the heart, and you've got yourself some trust issues all through the fabric of the film.
If the Council had been completely open with Anakin instead of keeping him at arm's length, he wouldn't have felt the need to rely on Chancellor Palpatine as a mentor.
The Council was open with Anakin, but Chancellor Palpatine is just a really good manipulator.
There are really two categories of power in Revenge of the Sith. There's the power of the Force—an immutable strength to manipulate elements and an ability to know things beyond your normal conception.
And then, there are your regular types of power—autonomy and the ability to make decisions and perform actions based on your own convictions. Anakin has a ton of one kind of power but not a whole lot of the other kind, which leads to some pretty disastrous complications.
When Anakin says that he wants more power, he is referring to the invincibility that comes with power in the Force.
When Anakin says that he wants more power, he is really referring to the power to make his own decisions and take control of his actions without having to always get permission first.