Biologists are the most common type of mad scientists in books, movies, and television in the last two hundred years. I never thought of starting my PhD in genetics as heading toward the dark side, but I may have to rethink that. I'll try not to create any horrific chimeras or crazy viruses during my lab rotations.
Though it makes sense, really. Biology is obviously the coolest scientific field, so we have more potential for fiction novels. Just think about it. What would a mad astronomer do? Look at stars maniacally? Yeah, biologists totally trump that.