SGANARELLE: But when a great lord is a wicked man, it is a terrible thing.

[Mais un grand seigneur méchant homme est une terrible chose.]

Molière (1622-1673) French playwright, actor [stage name for Jean-Baptiste Poquelin]

Don Juan [Dom Juan], Act 1, sc. 1 (1665) [tr. Waller (1904)]
    (Source)

(Source (French)). Other translations:

But a great Lord, a wicked Man, is a terrible thing.
[tr. Clitandre (1672)]

If a great lord is a wicked man it is a terrible thing.
[tr. Van Laun (1876)]

But if a great lord is also a wicked man, it is a terrible thing.
[tr. Wall (1879)]

But a wicked nobleman is a terrible thing.
[tr. Page (1908)]

But a great lord who is a wicked man is a terrible thing.
[tr. Frame (1967)]

But a wicked nobleman is a frightening master.
[tr. Bermel (1987)]

But a great lord who's a wicked man is a frightening thing.
[tr. Wilbur (2001)]

Sometimes rendered "What a terrible thing to be a great lord, yet a wicked man," though I could not find a good source for that phrasing, which is also attributed to Carlos Castañeda.