Zaroff especially welcomes rainford because the general-

Zaroff especially welcomes Rainsford because the general looks forward to the challenge that hunting Rainsford will pose. 

Zaroff is aware of Rainsford's reputation as a hunter. When Rainsford arrives at Zaroff's chateau, Zaroff tells him:

"Perhaps," said General Zaroff, "you were surprised that I recognized your name. You see, I read all books on hunting published in English, French, and Russian. I have but one passion in my life, Mr. Rainsford, and it is the hunt."

He is aware that Rainsford is a talented hunter and will be a challenge for him. To this point, Zaroff has been hunting sailors that become shipwrecked on his island (by his own design). He complains to Rainsford that they do not challenge him.  He is able to kill them too easily, and as a result, he is suffering from boredom.  Being bored by the activity that was once his passion is greatly unsettling.  Hunting Rainsford, who will be difficult to kill because he knows all the strategies and tools of a skilled hunter,  gives him hope that he will restore his passion for the hunt.