He thinks that life is the thing where you have to try everything. That is why he begins eating different strange stuff like flies, spiders, and even birds while they are raw and live.
Maybe Dracula liked this insanity because he made Renfield as his helper. Once our boy betrayed the vampires and Dracula killed him. So, yes, never lie to Dracula.
Before all those incidents Renfield was a patient of Dr.Seward in the asylum. Probably exactly because of this reason Renfield doesn’t have many lines in the book. Who cares what an insane person is talking about? He's described by the author of the text as a "lunatic" and a "madman," but there's definitely a method to his madness.
Nevertheless, to the end of his life, it seems like Renfield improves. It is clearly visible when he worries about Mina, and how Dracula hurt her. Exactly after this episode, he will be dead. When Dracula came to finish his work – to kill Mina, Renfield was trying to help her and stop him closing the way to the house. Exactly after that Dracula understood that Renfield isn’t a vampire anymore, he is a betrayer.
R.M. Renfield in the Essays