The sin was anger. God holds leaders and those who are righteous to much higher standards than ordinary people. That’s why the punishment was so severe.
There is an observation that Moses had a speech impediment so it could not be claimed that he beguiled people like a TV evangelist.
Really on the part of the speech issues. More like humility.
Yes, anger could had been the issue. The only person to be ever just in his anger, when He overturned the merchant tables is Jesus.