Silvio is not a hero, as, while he did not kill the Count, it was not for any heroic reason. It seemed to me that he did not kill the Count because he was more interested in making the Count react to him. if the Count died before doing that...he would not be satisfied. Thus, he instead delays taking the shot until the Count finally reacts. At that point, he has achieved his objective, and needs no more. Nothing about that is particularily heroic. However, it is not precisely cowardice either. Moreover, he claims that he did not fight because he didn't want to risk losing his life before taking his revenge on the Count. It was a tiny risk, implying he had other reasons not to get into that duel, but if it was simple cowardice, he would never have gone to the Count to take his revenge the second time around. Desire not to kill is also a possibility, but if that is the case, he is much too confused about such a feeling that it hardly merits giving him the title of 'hero'.