I mean, if you're asking a fellow to come out of a room so that you can dismember him with a carving knife, it's absurd to tack a 'sir' on to every sentence. The two things don't go together.
The excerpt humorously suggests that certain formalities become meaningless in extreme situations. When someone intends to harm another, such as dismembering them, the niceties of polite address, like using 'sir', seem utterly out of place. This highlights the absurdity of maintaining decorum in the face of violent intentions.
Wodehouse effectively contrasts etiquette with the gravity of the situation, showing how societal conventions can falter when confronted with stark reality. The dark humor serves to illustrate the nonsensical nature of trying to be polite while engaging in something so wildly inappropriate.