"Sadist" was mentioned earlier. Why we have "sadist" rather than "sadean" is somewhat more difficult to explain than most other "ist" endings, perhaps.

A "sadist" is "a person who is like the Marquis de Sade", as Father Steve has said, but a "sadist" may not know anything about the Marquis de Sade and his infamous practices. In this, "sadist" seems to be an eponymic anomaly. What Marxist would not know about Marx? What Calvinist would be ignorant of Calvin?

But, having said that, a "sadist" is one whose defining characteristic is not an intellectual interest in, or passive acceptance of, human perversity, such as one might find in a psychoanalyst [befitting an "ian" or "an" ending], but the active expression of that interest in acts of cruelty.

On this basis, "sadist", not "sadean", is the proper word to describe someone who is "like the Marquis de Sade".