The Bais Avrohom explains Rashi by quoting the explanation of tzadikim on the words “sur mei’ra v’asei tov”, turn away from bad and do good. They explain that when someone tries to do something good, such as to learn Torah or do mitzvos, the yeitzer hara knows that this will bring him to do teshuva for his past sins. Therefore, it reminds him of his aveiros in order to confuse his thoughts and make him feel sad, so that he will not do the mitzvah with full devotion and enthusiasm.
For this reason, when one does good things, he must simultaneously turn away from bad things, meaning that he must not think about the aveiros he did in the past so that he can do the good thing to perfection. Accordingly, Moshe asked Aharon: “Why are you ashamed?” He knew he was thinking about his sin of the past and he told him that he must not be concerned about it when he did the avodah. He said that by vanquishing thoughts of his past sin, he will be able to achieve full teshuva.
In a similar vein, the Tiferes Shlomo explains the pasuk (9:22): “And Aharon lifted his hands to the nation and he blessed them and he came down from making the Chatas, the Olah and the Shelamim” with the words of the Gemara (Pesachim 64B): “Ain maavirim al hamitzvos.” (Do not pass by mitzvos.) He explains that a tactic used by the yeitzer hara is to try to convince a person he is unworthy of serving Hashem. It tells him: “Why are you? You have committed so many sins. How do you have the chutzpah to daven to Hashem.”
The truth is, however, that Hashem hears every prayer, and He is merciful and compassionate to all people. The words “ain ma’avirim al hamitzvos” mean that no one should think about the aveiros he has done (the word “ma’avirim” can be understood as a reference to aveiros), at the time that he is doing mitzvos. Regarding this, the pasuk says: “Sur mei’ra v’asei tov.” This means that at a time when someone is doing something good, he should not think about the bad things that he did in the past. This is also the meaning of the words “and he came down from making the Chatas.” At the time when Aharon blessed Klal Yisroel, he did not think about their sins (chatas) of the past.
