Not because he was wrong to sack McGrath — though I think he was.
Not because he's a closet racist — I doubt he is, but, really, why would it even matter if he was? Let's say, for the sake of argument, that he doesn't like black people. So what's he going to do? Deport them? Lock them all up in special camps? Make it illegal to employ them? Even if he did have such private views, he's simply not in a position to make them into any sort of policy, and anyone seriously worrying about the truly terrible things that could befall black Londoners if a racist were to become Mayor is, frankly, insane.
No, it's because of the rationale for the sacking:
'We both agree that he could not stay on as my political adviser without providing ammunition for those who wish to deliberately misrepresent our clear and unambiguous opposition to any racist tendencies.'
Politics is, if nothing else, about understanding people's behaviour. Boris is correct that those who are determined to paint him as a racist would have continued to bring this up ad infinitum had McGrath stayed on his staff, but appears to be under the impression that, with him gone, they won't. This is such a fundamental and major failure to understand his political enemies that you have to wonder how he ever managed to win an election in the first place.
Yes, yes, I know: it wasn't Boris's decision, it was David "Bloody" Cameron's. But, look, I know he's only been in the job a couple of weeks, but he should be standing up for himself — and for his office. He needs to understand that, in London, he's actually more important than Cameron, party politics be damned. He's supposed to be running one of the world's largest cities, while Cameron, on the other hand, party leader though he may be, has the far less important job of asking the Government questions on behalf of the people of just one constituency. Boris has been elected to a position of wielding power; Cameron has been elected to a position of criticising it. For Boris to obey orders from Cameron is akin to Schwarzenegger running California according to instructions from John McCain. And his failure to understand that is not a point in his defense, but merely another reason why he's not fit for the post.
But you never know. Maybe he'll figure it out.