The issue with Muehller, and Rosenstein, is that they should have recused themselves.

If you have integrity, you never put yourself into a situation of where their appears to be a conflict of interest. Both of these guys have well-documented areas of conflict to the degree that they should have recused themselves (Rosenstein) or never considered the job in the first place. (Muehller)

The fact that we now know that Muehller was openly interviewing for the special counsel position at the same time he was interviewing with Trump for the FBI director position is just another breadcrumb that came out this week that wreaks.