According to Zaroff what was the worst part of the situation that occurred when the foolish fellow went into the death swamp

The worst part of the foolish fellows venture into the swamp was that Zaroff lost one of his best dogs.

I suggest, too, that you avoid the big swamp in the southeast corner of the island. We call it Death Swamp. There's quicksand there. One foolish fellow tried it. The deplorable part of it was that Lazarus followed him. You can imagine my feelings, Mr. Rainsford. I loved Lazarus; he was the finest hound in my pack.