Yes, it is.
When boiled down, good is basically thinking of others and how your choices impact other people. Evil is just the opposite.
Darth Vader: Whaa, the Jedi are old and stupid; they are just holding me back. It would be much easier for me to just kill everyone that opposes me rather than try and work things out.
Saruman: I don't need to listen to anyone, I am smart enough to use this Palantir and no one can corrupt me!
Cersei Lannister: Need I say more? lol
Not really.
Anakin Skywalker only became Darth Vader because he couldn't cope with the loss of loved ones. The loss of his mother, and her pitfull life, caused his soul to be vengious and receptive. He was heavily conflicted in choosing the dark side but his love for Padmé pushed him over, fearing she would die in chilbirth and believing the dark side had the power to prevent it. Darth Sidious (aka Palpatine or the emperor) spun a web which Anakin just couldn't avoid. This theme, fear for the loss of loved ones being his main incentive, continues till his very end, when he redeems himself by killing Sidious to save his son's (Luke's) life as his ultimate act. You can only say Amakin handled selfishly if you consider "love" selfish in essence. He certainly didn't hapily walk over to the dark side because he'd think it would be "easier" or because he felt "the Jedi were old and stupid and just holding him back".
Sauruman also didn't just fall to Sauron due to selfishness. He fell to the enemy because he opened his mind to the enemy. He was studiying the arts of the enemy and keeping track of the enemy through the palantir because he believed that in order to defeat the enemy mankind had to know the enemy. As the story progresses the corruptable power of Sauron was just to great for Sauruman to resist. He was however initially set on his gradual path of submission to Sauron's will because he believed he was acting in the common good.
Considering real world evil, in it's most ultimate form imho, think of Adolf Hitler. He genuinely believed he had the common interest and progression of the german people at heart. Megalomanial madness perhaps, but he didn't solely act on selfish interests.
Cersei Lannister is a good example of selfish interests yes.
So, is selfishness the essence of evel? Not really. It's a factor. But there are many factors that push people towards evel. Even love can push people to evil. The essence of evil is that it is in the eye of the beholder.