A mad scientist is a stock character in many a cheap sci-fi or horror flick. Usually dressed in a tattered lab coat, dishevelled dress shirt and slacks (garish tie optional), chemist or surgical gloves, and cokebottle glasses.

Maybe they got pushed into one too many lockers in high school. Maybe the university rejected their thesis. Again. Whatever the reason, they're a scientist who's gone gigglingly bonkers and began performing Unethical Experimentation on homeless people, alley cats, or even corpses, depending on their field of "study."

Usually, their experiments will be conducted without the rigorous documentation and comparison to a baseline that defines proper experiments. They may even perform an experiment on themselves.