We know that the yetzer tov can use its opposite, the yetzer ha-ra, for its own purposes, either by shelo lishmah, or as described in the article "Light from Darkness" (See p.180). We learn this from the saying: "You shall love Hashem... with all your heart-- with both your inclinations, the good and the bad.
It may not be realized that the yetzer ha-ra can use the yetzer ha-tov for its own purposes, too.
Say the yetzer ha-ra causes a person to sin, and the yetzer ha-tov then counterattacks by giving him conscience-pangs. The person suffers from the contradictions within himself. The yetzer ha-ra may tempt him to rid himself of the contradictions and silence the voice of conscience by self-deception and radical denial. This means that he decides to reverse his values and henceforth to consider the bad as good and the false as true. The yetzer ha-ra has succeeded in converting the demand of the yetzer ha-tov into a factor leading to denial and heresy.
This is the meaning of the saying of our Rabbis: "It is not enough for the wicked that they have not turned the yetzer ha-ra into yetzer tov; they have even turned the yetzer tov into yetzer ra'."
The lesson is that one has to look very carefully at the demands of the yetzer tov. One has to refine them, select them, and see that they lead promptly to practical results. If not, one might wake up one morning and find that the yetzer ha-tov has lost its voice.
It is all too easy to forget the true perspective and adopt the opposite view. This is the one that has many names--"practical," "realistic," "progressive"; but whose real name is nothing more nor less than: "false."
The result may be that the person finds he is --at least temporarily--at peace with himself. But may God preserve us from that kind of peace!
(Strive for Truth, Rabbi Eliyahu E. Dessler, volume 2 p. 95)