He invented an extraconstitutional legal standard for his obstruction investigation and acted, at best, in violation of the spirit of the special-counsel regulations.
His departing act was a public statement meant to influence the public debate in a manner inappropriate for a prosecutor, in part because the long public report he wrote that was also inappropriate for a prosecutor lacked clarity.
A hallmark of the Trump era is that the norm-defying president goads everyone appalled by him to violate norms. The former Marine and G-man Robert Mueller would seem least likely to fall prey to this dynamic, but here we are.