What is ironic about Zaroff’s statement: “Oh, you can trust me… I will give you my word as a gentleman and as a sportsman”?

This statement is ironic because Zaroff is claiming to be a gentleman, one whom the reader might predict is civilized and trustworthy.  Zaroff, however, is anything but civilized when he hunts other people on the island.  He claims to be a gentleman when he's a murderer in all actuality.