You'll never look at houseflies the same way again after Renfield decides they're a tasty snack.
Renfield, a patient in the asylum where Dr. Seward works, is a difficult character to get our heads around, simply because of his abnormal psychology. He's described as a "lunatic" and a "madman," but there's definitely a method to his madness. It's never made clear how Renfield found out about Dracula, or why he's so sensitive to the vampire's movements. Nor are we told where Renfield's strange obsession with becoming immortal by absorbing and consuming other lives came from.
By the end of his life, though, Renfield becomes more sympathetic—he seems sincerely to like Mina when she comes to visit (who doesn't like Mina?), and he's worried that she'll be hurt when Dracula comes, so he warns her to leave (although he doesn't tell her why). Then, after Dracula starts drinking Mina's blood, Renfield tries to stop him from entering the house again. What makes Renfield change his mind? Is it purely selfish, because he wants Dracula to make him a vampire, and not Mina? Or does he really pity Mina and feel sorry for having let Dracula into the house? In short, Renfield is something of a puzzle.
Of course, that's not surprising—Renfield is a puzzle to his doctor, Jack Seward, and we only ever hear about Renfield from Dr. Seward's journal entries. How might the story be different if we were somehow able to access Renfield's journal entries, or hear his point of view from another source?