Why was hunting beginning to bore Zaroff?

Zaroff was bored because the hunt was no longer challenging.

"Simply this: hunting had ceased to be what you call `a sporting proposition.' It had become too easy. I always got my quarry. Always. There is no greater bore than perfection."

The general lit a fresh cigarette.

"No animal had a chance with me any more. That is no boast; it is a mathematical certainty. The animal had nothing but his legs and his instinct. Instinct is no match for reason. When I thought of this it was a tragic moment for me, I can tell you."

Rainsford leaned across the table, absorbed in what his host was saying.

"It came to me as an inspiration what I must do," the general went on.

"And that was?"

The general smiled the quiet smile of one who has faced an obstacle and surmounted it with success. "I had to invent a new animal to hunt," he said.

"A new animal? You're joking." "Not at all," said the general. "I never joke about hunting. I needed a new animal. I found one. So I bought this island built this house, and here I do my hunting. The island is perfect for my purposes--there are jungles with a maze of traits in them, hills, swamps--"