The vampire Dracula is pretty unambiguously evil. The members of the Crew of Light, the group dedicated to destroying Dracula, are unambiguously good. Sure, each of them makes mistakes – they're only human. But their intentions are always good. In short, Dracula is a classic story of good versus evil. You know when you start reading the novel which side is going to win (the good guys!) – the question is how great the cost will be. How many of the good guys are going to have to sacrifice themselves in order to conquer the evil Dracula?
Because Dracula is clearly a traditional good versus evil narrative, readers realize from the beginning that Dracula will be defeated; the suspense is therefore not about whether the good will prevail, but about how many will have to sacrifice themselves to bring about the final victory.
Renfield's ambiguous status in the novel helps to complicate what would otherwise be an overly simplistic division of characters into "good" and "evil."