A number of commentators who are not simply apologists for Donald Trump have been arguing that the firing of Deputy Director Andrew McCabe by (supposedly recused) AG Jeff Sessions cannot have been a simple political hatchet job, because it was based on a recommendation of the Department of Justice Inspector General, a nonpartisan professional who was appointed to his current position by President Obama. I think they're making a straightforward logical error.
McCabe contends that he did nothing wrong. Maybe he's right about that. Let's assume for the sake of argument, however, that he's wrong. In other words, let's stipulate that if and when the report of IG Michael Horowitz is made public, it contains smoking-gun evidence that McCabe committed the wrongs that have been publicly alleged and that these are firing offenses, even for someone who is barely a day away from retiring with full benefits. Nonetheless, it is possible -- indeed, given Trump's very public campaign to discredit the Mueller investigation and anyone who could aid it, it is likely -- that the evidence contained in the IG's report was not the actual reason McCabe was fired.
The firing of James Comey closely parallels McCabe's firing.
Deputy AG Rod Rosenstein wrote a letter detailing how Comey's mishandling of the investigation of Hillary Clinton's emails--especially his public comments on the investigation--was a serious breach of policy warranting dismissal. Trump then fired Comey, initially claiming that he did so based on Rosenstein's report. But that was obviously just a pretext. As Trump himself soon boasted, he would have fired Comey without the Rosenstein recommendation. Why? Because of "this Russia thing."
Likewise, IG Michael Horowitz prepared a report detailing how McCabe's mishandling of the investigation of Hillary Clinton's emails--especially his authorization of comments to the media on the investigation--was a serious breach of policy warranting dismissal. Sessions, who had good reason to fear that Trump would fire him if he did not act against McCabe, then fired McCabe, claiming that he did so based on Horowitz's report. But that is likely just a pretext. Trump hasn't yet publicly boasted about it quite so explicitly as he did with Comey (although he has come close), but the most logical explanation for McCabe's firing--even assuming that he was fireable--is "this Russia thing."