You know the evil these people are, "Make us gods who will lead us" - Exodus 32:22-23
Aaron had a opportunity to stand up for God - he failed
He had been with Moses long enough to know better
God had commanded the Israelies not to make themselves any kind of idol - Exodus 20:4-5
Aaron could not find within himself to stand up to the people and claimed to be an innocent bystander
He failed both when he buckled under the pressure of the people as well as taking responsibility for his part of the sin