It kind of isn't. Being two faced and smarmy is a core part of the job description for any politician. That's how they get elected. The "prick" thing is maybe more debatable. Although I've never met a politician I didn't dislike.
He's not flip-flopping on an issue here or there. It's not details about how much gets spent in a particular department. It's the entire manifesto and vision. But I want to highlight that he was Shadow Brexit Secretary and vocally and forcefully pushed through a second referendum in that manifesto past a reluctant Corbyn (which IMO turned a bad result for Labour into a landslide loss). Like many things in that manifesto, a Brexit 2nd referendum wasn't just a small detail, it was a fundamental, deep statement about the future of the UK , its relation with its largest trading partner, and the meaning of the most important event in recent British politics (the Leave vote) --- and it was personally supported and pushed by Starmer in his official role. Which he has also gone back on fully and completely.
Can you imagine Blair calling for nationwide nationalisation of the commanding heights? Or for leaving the European Union?
This is fundamentally an empty man. (Which also explains his endorsement of collective starvation of Gaza - it was a sensible policy for that moment and to distance himself from anti-semitic Corbyn).
In the Indian elections recently concluded, my state saw a fight between two alliances of 3 parties each. Two of the parties can very roughly be analogised to Labour and Tories, and were indeed on opposite sides. The remaining 4 are literally factions of each other. To put it into, again very rough, British terms, it would be
Tory + Official SNP (most 2nd rung leaders) + Official Lib Dems (the founders' nephew and some 2nd rung Lib Dem leaders)
vs
Labour + Fake SNP (Sturgeon + most grassroots SNP workers) + Fake Lib Dems (founder + most grassroots Lib Dem workers)*
It was a fun election!
A successful candidate for the "Tory" analogue was a guy who went from SNP (before it was divided) -> Labour -> Tory, timing each defection for maximum chance of winning his seat! A guy joined Fake Lib Dems from Official SNP because he had a grudge against one Tory candidate, and he moved to that seat and beat him! After doing badly, half of Official SNP now wants to go back to Stuurgeon! The state chief of Labour jumped ship to the Tories weeks before the election, Labour won his pocket borough against his chosen candidate.
Despite the results being positive in my view, the politics of my state are an absurd farce; each candidate barely represents a party, let alone an ideology. I think Starmer's "two-facedness" is exactly as bad as this because of how fundamentally he has done a 180 on everything.
*calling them Official/Fake because our Election Commission declared that the factions allying with the Tories were the real ones in both cases)
** also in this analogy I was torn between calling it UKIP or SNP, it is both, was a far-right Tory ally, but is also a regionalist party.