The Righteous Dao can be considered one of the major "sides" of the Dao. The Righteous Dao does not refer to a specific power or law, it simply classifies a cultivator as "overall good". A cultivator can be placed under this classification even if they are mostly neutral but have a little good karma. Although there are indeed spiritual techniques that follow "purity and righteousness" and special cultivators who cultivate the "righteous path", the Righteous Dao itself is still a generalization.
Some self-proclaim themselves as a Righteous Dao cultivator even if their actions do not align with the majority of Righteous Dao cultivators. In the end, the interpretation of the Righteous Dao falls in the hands of the cultivator. As everyone has their own meaning of right and wrong, there is no true "righteous cultivator" and there is no true "evil cultivator."