Ah, good questions. First, where does the author say that one of the characters lived life in a minute?
My version of the story says, regarding Rainsford, that "He lived a year in a minute." This means that his senses were so heightened by the adrenaline rush of one man hunting another that it feels like that minute is full of a whole year's activities. It also refers to how time feels like it slows down at moments of crisis.
As far as still being a beast at bay, that has several meanings. Most simply, it means that the "game" didn't end when they'd originally planned: the hunt is still going on. On a deeper level, it refers to Rainsford's having found a more dangerous inner self. He's both warning General Zaroff and admitting how much this "game" has changed him.