To be fair I don't think it is in this case, she's done the right thing for the right reason.
At Wednesday's Prime Minister's Questions, the Labour leader hit back, accusing the PM of fuelling conspiracy theories with his remarks.
But Mr Johnson replied that Sir Keir had taken "full responsibility for what had happened on his watch" as the DPP.
By Thursday, Mr Johnson appeared to have backed down, telling reporters he had not been talking about Sir Keir's "personal record" when he was DPP, adding: "I totally understand that he had nothing to do personally with those decisions.
"I was making a point about his responsibility for the organisation as a whole."
But in her resignation letter, Ms Mirza's letter says he should have gone further and apologised to Sir Keir.
The political aide - who has worked with Mr Johnson since he became Mayor of London in 2008 - said the PM had been "wrong" to make the initial remarks, adding: "There was no fair or reasonable basis for that assertion.
"This was not the usual cut and thrust of politics; it was an inappropriate and partisan reference to a horrendous case of child sex abuse.
She added: "You tried to clarify your position today [Thursday] but, despite my urging, you did not apologise for the misleading impression you gave."
Ms Mirza began her career in the arts sector before becoming a cultural adviser to Mr Johnson as London Mayor.
She then followed him into No 10, and was named the director of his policy unit in 2019.
In 2020, the PM named her as one of the five women who had influenced and inspired him the most, describing her as "capable of being hip, cool, groovy and generally on trend".
A Conservative Party insider described Ms Mirza as "Boris' brain and longest-serving adviser", adding: "For her to depart at such a difficult time and in such a publicly critical way is simply devastating for the PM.
"It sends a strong signal to the Tory Party that even those closest to the PM have now lost faith."
A former Conservative adviser also told the BBC's political editor, Laura Kuenssberg, there was no way for the PM to "spin" her departure.
"It's a devastating blow," they said.
"First, because he's lost one of the few people inside No 10 who knows him really well and is an ideological ally. And second because the criticism, which is damning, is from within.
"And you simply can't paint that as nonsense from the media or his political opponents."
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