Barr said investigation should have "stopped right there" when Mueller decided he could not prosecute - signalling Barr thinks that the rest of the investigation was illegal after that decision was made -- AND THAT DECISION WAS MADE AT THE OUTSET. Barr is coming after Mueller.
Mueller is prepared and notes that while a POTUS cannot be prosecuted, "a criminal investigation during the President's term IS PERMISSIBLE."
Mueller does not agree with Barr's decision to DECLINE to prosecute, believes correct answer is to NOT make that decision during term of office. Barr gave that a big FU.
Mueller also does not agree with Trump's lawyers that he is exempted from obstruction statutes. Says suborning perjury, intimidating witnesses, or fabricating evidence raise no separation of powers issues.
Here's a shot over your bow, Trump. Bribery (which is not covered by the Mueller report but of which there is a lot of smoke/fire) CAN BE INVESTIGATED. Tick Tock, Trumpers.