What is Rainford's first impression of General Zaroff?

THat depends where in the story you mean. Initially he thought Zaroff was a pretty good host. He respected Zaroff's hunting experience and taste for fine things. Half way through dinner Rainsford discovers the Zaroff is actually a twisted murderer who hunts people for fun.

He warmly welcomes Rainsford, but Rainsford can tell there is something strange about Zaroff.