Sefer Divrei Bina (Biala) states that this pasuk can be understood to be telling the Kohen Gadol and all other tzadikim how to rebuke Klal Yisroel. He explains that when a Jew complains to a tzadik that he lacks parnassah, children, etc., the tzadik may know that the source of this individual’s problems is his own sins. Since he was found lacking in his ruchnius, he is unable to receive the goodness he desires. The tzadik could chastise him for blaming Hashem for the problems he himself caused. However, if he would harshly criticize the man, the man would fall to despair and would give up hope.
Therefore, giving harsh tochacha at such a time is considered “tamei”, as it could lead the individual to engage in more impure actions since he has lost hope of ever improving himself.
The pasuk is saying that kohanim, i.e., leaders of the nation, should be told: “Do not make a nefesh in your nation impure” by saying such things to them. Instead, people should be spoken to gently and told that they can do teshuva and thereby merit Hashem’s kindness and mercy.
