Bible.com Keep The Feast, day 24
And Moses said to Aaron, “What did this people do to you that you have brought such a great sin upon them?”
I have wondered the same thing. What is the difference in how Moses is looking at things and how Aaron is? A few verses earlier, God says to Moses (paraphrasing), “Get out of my way so I may destroy them. Then I'll make a nation out of you!” Moses declines. He argues for the people, standing in the gap (Ezekiel 22:30) before the LORD. He thinks nothing of the fact that God might replace all of Israel with just the descendants of Moses, which would have elevated his own position considerably. (Weirdly, we have to assume this is God's plan from the beginning — for Moses to prove his loyalty in this moment; for Moses to dissuade God from changing the whole course of history.)
Contrast this with Aaron. “Hey man, make us some gods!” A little peer pressure, and Aaron caves. He has no love for God; he just wants to continue leading the people as their priest. It's like the U.S. Congress; all he's worried about is re-election.
Also, Moses punishes the people for their sin, but he blames Aaron. The leadership lesson is clear: you are responsible for the things you allow, condone, and enable your people to do. Those in authority are judged to a higher standard (James 3:1). (And, duh, don't make or worship any idols… including the idol of your own position.)