Darroch resigning isn't surprising. What's surprising—or perhaps not—is the way Johnson threw him under the bus. He could—and should—have refused to go along with the leaker's agenda, dodging questions about Darroch's future job prospects, instead keeping the spotlight on the extremely worrying precedent set by this targeted attack and on Darroch's long history of faithful service to the UK. You'll note, btw, that the US's ambassadors don't easily get fired over offensive conduct, and neither do Russia's. The UK's civil servants saw Johnson demonstrating extremely poor leadership, as many current and esp. former civil servants have stated.