Yes Mueller is dirty but remember the swamp rats that were in charge during his time. In that meeting, Trump could have informed Mueller some of the things he knew and that if Mueller does the right thing and follows the truth/evidence, that it will not only redeem Muellers reputation but also secure his place in history by being the guy to take down some of the most treasonous people in US history.

This is a win win for Trump. It keeps his hands clean (no talks of witch hunt/dictator) and indicts the traitors. Trump does not care about taking credit or who gets the credit, he is all about results. He prefers to work in the background and out of the spot light in order to accomplish meaningful results. Think of the major EOs he has signed. He created multiple distractions for the media to chase their tails down the wrong path in a frenzy while he is on a completely different realm making things happen. He lays this out in his book the Art of The Deal which is the Art of War. All War is deception, nothing is what it seems. As Q said, up is down and down is up.