They're not resigning because they've suddenly realised he's a wrong 'un, they're leaving because he's become an electoral liability. But I suspect you know that - it's how politics works essentially everywhere, including the United States, for parties of all ideological persuasions.

If not, you might want to start with this short article in the Spectator[0] (one of the Conservative house journals, which Johnson used to edit) by Toby Young (a Tory) fifteen years ago (after his rise to prominence, but before his rise to power). The man's a scoundrel and his party have always known it. And even generally honest politicians aren't going to write "We gave you a go because we hoped you'd win the election, but we underestimated just how unpopular you'd get - please go now" in a resignation letter.

[0]: https://www.spectator.co.uk/article/having-your-cake-eating-...