The mood generated in the first few paragraphs of "The Interlopers" is one of isolation and even a sense of antagonism.
Remember that "mood" is is the feeling generated within a work, while "tone" is the writer's or narrator's attitude.
There are several ways in which Saki generates the mood in this story, the first being the description of the setting. Saki describes "a forest of mixed growth somewhere on the eastern spurs of the Karpathians. Then Ulrich "patrolled the dark forest in quest of a human enemy." And finally, "the narrow strip of precipitous woodland that lay on its outskirts." All these descriptions generate these feelings of isolation and the characters being on their own.
Then Saki generates the feelings of antagonism as Ulrich was "watching and listening, as though he waited for some beast of the woods..." Then the narrator describes the background of the feud between the two men. The disputes are also described in colorful language like "they had thirsted for one another's blood."
The story is one that is defined by its first few paragraphs. The rest of the story continues in this mood until the end when the men find themselves as allies because of a common enemy—nature.