satan (associated with saturn) meant something like "obstacle". everyone had his own satan. someone who incites him to do bad things, a bad habbit, an addiction, anythinghe should overcome. only later satan became a figure of his own.
6 is the number of man, and his weakness. greed, egoism, egocentrism, all the materialistic desires, and i believe of this plane existence in general (the earth is tilted 23,4 degrees, or if you count from the other direction 66.6 degrees, life, on this planet at least, is made of carbon, the 6th element, with 6 protons, 6 neutrons and 6 electrons. 6 directions in the material realm, north, south, west, east, up and down, 6 sides of the cubes. Think About how molecules and icecrystals form in hexagonal shapes. saturn being the 6th planet with hexagon on its northpole). 666 is going over the top with the materialistic desires, pursuing and desiring only those and making you follow the path of satan.
The black cube of satan is a symbol for this material, 3 dimensional realm, and the time we fear is running out, which will let us die, and which causes us to do harm to others in order to lead a better life ourself. by this the black cube also symbolizes time and death without either we wouldnt have these earthly desires causing us to do evil.
The 666 could also stand for the dimensions of the cube, 6x6x6, entrapping us in the 6, our weakness and temptations.
Our sole puprose, as i believe it, is to overcome (our) satan, proof we arent easily corrupted wether satan is just a symbol or a real entithy. So while this world isnt evil itself, this realm of existence kind of makes the corruption a lot easier. When people worship satan, they dont worship some deity or enthity. they worship this material world, the materialistic egoistic desires, and themselves.