Why does Zaroff think Rainsford is “droll” and “naïve”?

Zaroff thinks it’s foolish and old-fashioned that Rainsford values human life even after fighting in the war.

The general laughed with entire good nature. He regarded Rainsford quizzically. "I refuse to believe that so modern and civilized a young man as you seem to be harbors romantic ideas about the value of human life. Surely your experiences in the war--"
"Did not make me condone cold-blooded murder," finished Rainsford stiffly.
Laughter shook the general. "How extraordinarily droll you are!" he said. "One does not expect nowadays to find a young man of the educated class, even in America, with such a naive, and, if I may say so, mid-Victorian point of view. It's like finding a snuffbox in a limousine. Ah, well, doubtless you had Puritan ancestors. So many Americans appear to have had. I'll wager you'll forget your notions when you go hunting with me. You've a genuine new thrill in store for you, Mr. Rainsford."