John MacArthur, Matthew 1-7, Moody, 1985, p. 435.
The very nature of self-righteousness is to justify self and condemn others. In so doing people play God, because they judge themselves on the basis of their own standards and wisdom. Self-righteousness is the worst of sins because it is unbelief. It trusts in self rather than God. It trusts in self to determine what is right and wrong and to determine who does what is right or wrong. Self-righteousness claims to be both lawgiver and judge, prerogatives that belong only to the Lord.
John MacArthur, Matthew 1-7, Moody, 1985, p. 435.