Exode 32:21

And Mosheh said unto Aharon, What did this people to thee, that thou hast brought upon them a great sin?

WHAT DID THIS PEOPLE UNTO THEE, THAT THOU HAST BROUGHT SO GREAT A SIN UPON THEM? “How many pains did you endure, and how much suffering did they inflict on you, before you brought this sin upon them?” This is Rashi’s language. But it does not appear to me to be correct. For this transgression [of idolatry] is of the kind for which one must sacrifice one’s life and submit to death rather than transgress it [and from Rashi’s words it would appear that if Aaron had suffered much it was permissible for him to make it]. Perhaps Moses told him so in order to magnify his guilt.
The correct interpretation appears to me to be that this is like the verse, What have I done? what is mine iniquity? and what is my sin before thy father, that he seeketh my life? Moses is thus saying: “What hatred did you have for this people, that you have [almost] brought about their destruction and annihilation?” Moses told him this because Aaron had served them in the function of one who reproves and atones for them, and he should have had compassion and mercy on them [and should have made them desist from their course of conduct]; thus the meaning is: “You conducted yourself towards them as an enemy who desires to see their calamity, when they had neither sinned nor transgressed against you.” Now the proper order would have been for Moses to blame him firstly for the sin which he himself had done, and then to charge him for the sin which he brought upon the people, saying: “how did you do this great sin against G-d, causing also many people to trespass, and bringing a great sin upon them?” Moses, however, in his humility showed respect towards his elder brother, and only mentioned to him the stumbling of the people. It is possible that Moses’ heart was steadfast, trusting in the righteousness of his brother [and he assumed] that his intention was not a bad one, [and therefore he did not reprove him for his own conduct]; but for the people’s guilt he did blame him, for he should have reproved them, and thus they stumbled because of him, — and Aaron replied that they deceived him with their words.

הבאת עליו חטאה גדולה, a reference to the display of merriment in serving the golden calf. Moses criticised Aaron for having proclaimed the feast which led to this rejoicing over the golden calf instead of instead of being a feast for G’d. On account of this he was forced to entreat G’d for an additional measure of mercy for His people. This is also what he meant when addressing the people saying: אתם חטאתם חטאה גדולה “you have committed an exceedingly great sin,” (verse 30) Also, when appealing to G’d, he commences by saying that the people had been guilty of an exceedingly serious sin (verse 31). This theme returns once more when G’d speaks of עון-פשע-וחטאה, sins of three different levels of seriousness in Exodus 34,7. In 34,9 Moses begs for forgiveness for two of these categories of sin, עון and חטא, not daring to ask for forgiveness for פשע, deliberate taunting of G’ by one’s sin. Moses. who had shown understanding for the fact that Aaron had been under great pressure in acceding to the making of the golden calf, cannot understand why he lent his hand to heaping additional sin on the people by having declared a public holiday. He had not been under pressure to do that and should have known that the people would understand this as an invitation to celebrate the new leader, replacing Moses.