You can not even define evil without discussing what is good. Therefore, the two are inseparable. One can not understand darkness without experiencing light. One cannot appreciate day, without having night. Opposites compliment, define and complete one another. To destroy all evil in the world means there would be no good left to appreciate, therefore the Satanist does not worship or disrespect either quality! We value ourselves and nature instead and venerate opposition in balance!
The definition of good and evil is subject to change because it exists only as a perception. "Evil" is a label we assign according to our mood swings to those who are politically incorrect. The ascended masters of Eastern Philosophy become the fallen angels of Western Philosophy. One man's "demons" are another man's "saints." Theologians create gods and devils in man's image, then conflict between them to divide the heart and conquer the soul! They pit spirit against matter, then appoint themselves position of supreme arbiter, divine interpreter of right and wrong, good and evil.
"Good" are the things we like. "Evil" are the things we dislike. It often depends on who or what you are. As Diane Vera says, if you're a mouse, cats are evil, but to the pet owner, cats are heavenly. Therefore, evil is not an essence - it is a value judgment. From a Christian point of view, Satan is "evil" because he represents the savage instincts they are attempting to hide or suppress. Satanists are truth seekers who desire to peek behind such labels.
Of course, there are certain ethical wrongs, but they are set by a society or culture so that all can live harmoniously. Examples of these would be thieving, disrespectful behavior and killing or abusing an innocent.
A chief attribute of the trickster god is the role of Devil's Advocate. The Satanist illustrates "opposition in balance," wielding contradiction like a scalpel in the hand of a skilled surgeon. Our objective? Closure to this unnatural dichotomy, this madness you call "good and evil." Bringing flesh and spirit together. The establishment of a New World Order, free of guilt, self-loathing and shame, where every man and woman is the god of his or her own life. We look upon religion as the starter of wars and the breeder of hate and intolerance. We believe that people are intelligent enough to instill moral and ethical codes into a societal structure without a threat of an imaginary supernatural being wreaking havoc on an offender.
source: Satanism Central - Do You Worship Evil?
Although not originally a satanic symbol, the Yin Yang is a symbol adopted by satanic culture, because it does visualize that belief that is commonly held.
This symbol represents the state of everything as the circle. It shows an intricate balance between the lightness and darkness, good and evil, the yin and the yang. And in the light part there is a spot of darkness. And in the dark part there is a spot of light. This is how satanists view the world, as a balancing of the two forces that move the world. Some of it we might like, others we don't. But even those parts that we don't like are there out of necessity. And as evil and dark as they might seem, there is a silver-lining somewhere.