The human mind works on the basis of associations and whenever there is competition we instinctively tend to take sides and futhermore exaggerate the difference in such a manner that one side is good and the other is bad. Even judges fall into this trap and do not maintain an equanimity before the law. In many board games of a serious nature we therefore make two sides, one black and the other white. That clearly identifies the groups and sets the pattern for the game. Black must be destroyed.
We have as rather weak-minded human creatures done the same thing. We establish God and Satan, Buddha and Mara, and a host of other antagonistic pairs. We place Identity on one side as evil and the Natural self on the other as the just and natural. We have also Samsara and Nirvana, but is this division into two parts really so?