Ivan's behavior towards Rainsford -- towards everyone except Zaroff -- is threatening at best. Rainsford, washing ashore on the island, is surprised to find a modern chateau built in the forest; he knocks at the door:
The first thing Rainsford's eyes discerned was the largest man Rainsford had ever seen -- a gigantic creature, solidly made and black bearded to the waist. In his hand the man held a long-barreled revolver, and he was pointing it straight at Rainsford's heart.
(Connell, "The Most Dangerous Game," classicreader.com"
Rainsford cannot communicate directly with Ivan, as the larger man is deaf and mute. However, Ivan defers to Zaroff, being indebted in some way. Ivan enforces Zaroff's will where the more "civilized" man does not wish to get his hands dirty; he also served as "knouter," or flogger, to the Czar in Russia. Ivan never shows any emotion, or shows any weakness. Rainsford is cautious of Ivan and, knowing the large man's strength, does not wish to fight him directly. Thus, Ivan can't be said to have acted "disarming" in any sense of the word, with the possible exception of the more sardonically literal.