The problem with Labour MPs now, in my view, is that the leadership is wayward and unsure. Nobody really knows what Starmer’s political principles are. He got the job by not being Corbyn (though he was happy to promote Corbyn, before he turned on him).
By contrast, Blair’s route to power was clearly mapped out - OMOV, ditching Clause IV, clear intent to listen to business, sceptical about the unions, realistic in foreign policy (the later Iraq war did for him, I recognise), a powerful Trot hunting and expulsion operation. And so on.
Starmer’s MPs are all suspicious of his politics - too right, too left, take your pick. Without any personal qualities and a complete loss of authority, he’s just become a hate figure.
All in all, I don’t think Labour MPs are any more small minded and vindictive than they were before, they’re just on the loose now and see no reason to shut up and toe the line.