what is the best evidence and quotation which let zaroff aware of his current viciousness ?

I'd say when he expresses his excitement at hunting people.

"I wanted the ideal animal to hunt," explained the general. "So I said, `What are the attributes of an ideal quarry?' And the answer was, of course, `It must have courage, cunning, and, above all, it must be able to reason."'

yes true (y) , but in conclusion do you mean explains how he obtains the men he hunts ? for me i think when he said "cossacks are savage and that he is cossack "