From what I hear from people around the Corbyn effort it was mostly spectacular naivety. Most people bought the ‘prominent human rights lawyer’ narrative. He was brand new (and almost certainly a plant) and had worked hard not to give away anything about his own political instincts.

He was selected for a safe seat in 2014 right after receiving the Knighthood of the Order of Bath - a fancypants establishment military honor basically that can also sometimes go to people in the civil service - and immediately got in during the election in 2015.

Also in 2015, a small group of people, including a number on the right, asked him to stand in the same leadership election as Corbyn, which he refused supposedly due to his concern over a lack of experience (something he didn’t show less than a year later, but we’ll get to that).

People were watching him to see where he put his support and he went for Andy Burnham whom the left would like or at least live with if it wasn’t for Corbyn’s surprise surge. It’s clear now that his politics - to the extent he has them - are nothing like Andy Burnham’s far from great but genuine socdem instincts, but at the time he was playing it very close to his chest.

I’d argue that he wasn’t even there to be a wrecker for Corbyn, but instead to at least insert himself into the potential Burnham leadership and ensure it stuck to the authoritarian establishment line (or possibly go full wrecker if needed). Corbyn’s surprise victory just suddenly upped the importance and perceived threat to the state.

Despite his apparent concerns about a lack of political experience just months earlier, he took the position of Shadow Minister for Immigration when offered it by the Corbyn leadership. They saw a Burnhamite, soft demsoc with relevant legal experience (despite it often behing draconian deportation of harmless offenders) as a good fit and a way to solidify the leadership up with the (they assumed) friendly but moderate middle of the party machine against the ghoulish right of the party who were almost certainly going to make life difficult for them.

He then joined the attempt to overthrow Corbyn’s leadership in 2016 when it had gained momentum, but was careful not to be one of the first. Whereas others were just nakedly anti-democratic and anti-left, he was smart enough to stick to the narrative of it being about Corbyn’s lukewarm support of Remain in the Brexit vote being a disaster for the results (a faintly absurd narrative, but the one that had the most play in the media and political circles).

He then immediately segued that into a position in the Corbyn admin as minister for Brexit. The Corbyn camp thought they had to sure themselves up on Brexit in order to head off more challenges, so tactically appointed who they thought was a soft-left Burhamite who was just an arch remainer.

They were wrong and had just let the fox into the henhouse and as soon as he was in Starmer starting wrecking, creating impossible, inconsistent Brexit positions, then counteracting them in public, and using other people in the press to keep his hands clean as he leaked and briefed against the leadership.

Despite the wrecking specifically on the issue of Brexit and the chicanery behind the scenes he kept his hands just clean enough to not be seen as the right-wing plant he was by the less engaged and cynical party members in the post-Corbyn leadership election.

Once again he positioned himself as continuing the social and economic justice agenda of the Corbyn leadership without all the problematic baggage and with a more moderate, press-friendly coat of paint. And once again he tried to keep his hands clean while people in his camp viciously and libelously went after Rebecca Long Bailey, the candidate that actually believed the things he professed to continue supporting, including having his friends in the media further damage the public perception of the party by whipping up a brand new, bullshit anti-semitism scandal to brand her as a looney Corbyn racist.

All the while anonymous corporate and private donors were pouring millions in dark money - apparently more than in any other Labour leadership race before - into his leadership campaign. The sources of that money we still mostly don’t know and he refuses to reveal despite the fact that it couldn’t possibly matter now electorally.

Since his leadership win he’s systematically purged everyone even soft-left (while welcoming actual bigots spouting hate speech and genocidal language back) that refuses to go along with the most right-wing agenda the Labour Party has ever had while not just abandoning but actively admonishing and opposing every left-sounding pledge he paid lip service too; peaceful protestors should be locked up, immigrants deported in policies to the right of the Tories, the NHS completely privatised, trans people legally and publicly attacked to make their lives as hard as possible, anti-racist activism prosecuted and dismissed as a fad, unions defanged and strikes opposed, government informants and intelligence assets given immunity from prosecution even in case of rape murder or torture, and on and on and on…