Such behavior would seem to manifest an error of reasoning only if the person also has a commitment never to act unethically and somehow believes that she is acting in accordance with this commitment. Most people have no such commitment. They are ready to act unethically in certain situations and, when they do, there is nothing wrong with their reasoning.
I don't think that people thus acting unethically are always emotionally indifferent to unethical behavior. They may emotionally enjoy the thrill of doing something unethical. Or they may be disturbed by their conduct, albeit not disturbed enough to avoid it. For example, someone finds her neighbor's wallet with $4000 in it. She is emotionally upset by the idea of stealing the money, and she would refrain if the amount were much smaller. But with such a large amount she decides to accept some emotional distress for the sake pocketing the cash. No emotional indifference here, and also no error of reasoning.