Politico had an interesting (although surprisingly long and detailed given how bloody soon they put it out after Comey got the sack) examination of the history of Comey: how he ended up alone, with no allies, and in a position for Trump to finish him off. In particular, they described him making his actions before the election as under the impression that Clinton was definitely going to be President, and so to make his hands appear clean (not hiding any sort of Clinton wrongdoing); people close to him expected that the worse case scenario was Clinton would become President, be really pissed at him, chew him out, and then he'd explain to her that it was her doing that caused this problem. Of course, it didn't end up that way, and Comey went down as the Man who Moved an Election.
Regarding Mattis and McMaster, frankly, I think that's neither here nor there. Those were both McCain wing picks, sure. But the next head of FBI is what matters, and the sheer level of, oh what's the word. the scrutiny that will be placed on them and the optics of their decision and the Senate confirmation will be hugely important. Either way, Neo isn't wrong here. Trump does have a tendency to appoint more DeVos's than McMasters. So given that, and the no doubt impressive scrutiny of his decision, it will be interesting to see what happens next.