"Renfield regrets following Dracula's orders, but he's too weak to resist" is how the character is written in the script, and is how Frye played the part, as is evident when the scenes in the movie are reassembled in the order presented in the script.

"Dracula gives him tasks to do but the last remaining drops of sanity and morality prevent him from enacting" is an impression gleaned from the jumbling of scenes resulting from Universal's eleventh hour re-edit of the film. It has nothing to do with Frye's performance, and to the majority of viewers Universal's re-edit has apparently harmed their perception of Frye's actual talents.

I wouldn't expect anyone who hasn't read the script or seen the re-cut to appreciate the difference. If you enjoy the film in its butchered state, and have no problem with the muddle that Universal has made of Frye's performance (and to a somewhat lesser extent, that of Helen Chandler), that's fine with me. But you really have no idea of what you're missing until you've seen the movie re-cut to match the shooting script.

There is true gold just barely hidden in the release version of the film, and it isn't that hard to uncover once the pieces are put back in their appropriate order.