The third reaction on reading Alistair Darling's memoir, after enjoying the quality of the writing and the nicely understated humour, is to wonder again how Gordon Brown was ever allowed to be Prime Minister.

Of course, we knew the main points of the story at the time. It was no secret that Brown resented Tony Blair's seizure of the Labour leadership in 1994, or that Brown was impatient to succeed him from 2004, or that towards the end there were policy differences between them. It became known, especially during Brown's three years as Prime Minister, that he could be rude, difficult and bad-tempered.

With each successive diary and memoir, though, our knowledge of Brown's unreasonableness advanced another notch. Each advance was surprising, but small, and produced diminishing returns of outrage, so it was easy to lose sight of how far we had come. Now we have to look back and conclude that his behaviour should have ruled him out of high office.

This is a strange effect of the erosion of the 30-year rule. Even The Richard Crossman Diaries, which started the trend, were not published until after his death, and five years after he left office. Each volume came out at least seven years after the period covered. That was the 30-year rule brought down to seven, if you will. The Alastair Campbell Diaries cut it to 12 days, coming out less than two weeks after his boss stepped down – although the full truth about Brown waited for the unexpurgated versions, of which there is still one more volume to come.

The third volume of Campbell's "complete" Diaries, published in July, recounted one appalling example of Brown's conduct after another – and still only took us to 2001, four years into Blair's 10-year stint at the top. Because there were so many, and because we had already got the basic idea – Brown was a monster – much of it went unreported. Some of the accounts of Brown being monosyllabic or childishly unhelpful when asked direct questions in meetings are pure John Cleese: so embarrassing they are not funny. Campbell tells of a discussion about Europe in 2000, when Peter Mandelson said that "we had to be more positive" and "Gordon literally turned away to look at the wall".

The truth about Brown does not reflect well on him, but it also raises further questions about the people around him. We started to ask them when Blair published his memoir last year, in which he said, for example, that he felt Brown threatened to use the loans-for-peerages scandal against him. Blair's book was like an Agatha Christie novel, in that it revolved around a riddle. The question that ran through it was not "Why didn't they ask Evans?" but "Why didn't he sack Gordon?"

And he gave an answer of sorts. It was not particularly principled or heroic, and therefore quite convincing, which was that Brown would have been more of a threat on the back benches, from where he would have succeeded in bringing Blair's time as Prime Minister to an earlier end. Well, possibly.

Then there are the questions for others in the Labour Party: How did Brown succeed unopposed to the leadership? And why did the party not change leader before the 2010 election? We do not need Darling's book to tell us that the hopes many in the party had that Brown would operate differently once he was in the top job were quickly dashed – although this weekend's revelations add brushstroke detail and depth to the picture.

John McTernan, who had been Blair's political adviser and who worked at the Scotland Office until last year, tells how Brown replicated his own broken relationship with Blair in his dealings with his own Chancellor. Brown wanted the 2010 election to be about "cuts versus investment". In one meeting of advisers from Nos 10 and 11, McTernan asked: "Surely the real choice is between our cuts and theirs?" He wrote in The Scotsman last week: "This was dismissed, but later No 10 issued an edict to Darling's staff. While they hadn't supported my line, they had rolled their eyes while No 10 staffers were talking."

Darling, too, managed to replicate one important feature of the dysfunctional relationship with No 10 of his predecessor, which was that he made it hard for the Prime Minister to sack him. Thus he was able to see off Brown's feeble attempt to appoint Ed Balls as chancellor, which has already been recounted by Peter Mandelson in his memoir – Brown asked Mandelson to find out how Darling would react to the possibility.

It would seem that Darling also learnt something else from earlier predecessors about score-settling. There are parallels between Darling's revenge and the turning of Geoffrey Howe, himself a former chancellor, on Margaret Thatcher, who had once been his ally. Darling and Howe were known for restraint in their public demeanour; both were urged to defend themselves by grudge-bearing wives. But the differences are more important. Howe took his revenge in the Commons, when his grievance was still hot and his tormentor could be brought down. He invited a leadership challenge and in weeks she was gone. Now that was a score settled: Darling's book is more of defeat excused.

The portrait of Gordon Brown that emerges from the memoirs and diaries is so bad that it can't all be his fault. Blair, Campbell, Mandelson and Darling stopped short of doing something to stop Brown when they had the chance. David Miliband and Alan Johnson, who never said a bad word about Brown but who could have sought the top job, chose not to do so. In my view, either or both should have done.

No doubt Alistair Darling thinks he could have done better as Labour leader at the last election, and I would agree with that too. But none of them did what had to be done, so it is not much use arguing now over who was right and who was wrong.

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