Which textual evidence from “The Interlopers” supports the conclusion that hatred feeds on itself?
"But the game for whose presence he kept so keen an outlook was none that figured in the sportsman's calendar as lawful and proper for the chase; Ulrich von Gradwitz patrolled the dark forest in quest of a human enemy."
"The descending twig had slashed the skin of his face, and he had to wink away some drops of blood from his eyelashes before he could take in a general view of the disaster."
"The feud might, perhaps, have died down or been compromised if the personal ill-will of the two men had not stood in the way; as boys they had thirsted for one another's blood, as men each prayed that misfortune might fall on the other…"
"'I never thought to have wanted to do other than hate you all my life, but I think I have changed my mind about things too, this last half-hour.'"

"The feud might, perhaps, have died down or been compromised if the personal ill-will of the two men had not stood in the way; as boys they had thirsted for one another's blood, as men each prayed that misfortune might fall on the other…"

Explanation:

In "The Interlopers", Saki (Hector Hugh Munro) deals with two families who had been at war since many generations back. The two families had been disputing about the land currently owned by the Gradwitz family, after Georg's family lost the lawsuit. This plot of land had been the bone of contention between them, lasting and going on for many years.

The characters of Ulrich von Gradwitz and Georg Znaeym were stuck in a trunk of a tree during the massive windstorm. Though they could have easily depended n each other at times of troubles, their decades long family feud had stopped them from making peace. Their pride and ego made them stubborn, refusing any chance of reconciliation.

The lines

"The feud might, perhaps, have died down or been compromised if the personal ill-will of the two men had not stood in the way; as boys they had thirsted for one another's blood, as men each prayed that misfortune might fall on the other…"

is the sure proof of the conclusion that hatred feeds on itself. Their feud ends permanently with the death of the two men, killed by wolves.