The original charge was to conduct a
counterintelligence investigation into Russian dabbling in the election. There was no crime alleged or indicated. There still is no crime alleged. There was and is no reason for Mueller to investigate Trump at all, which is why early on he was assured that he was not a target.
Now in the fetid ethical swamp that the legal business has become, none of that necessarily holds, and any of it can be ignored at will. Mueller has authority from Rosenstein to go rogue and yes the fact is, it truly is a witch hunt.