Make yourself an honest man, and then you may be sure that there is one rascal less in the world.

Thomas Carlyle (1795–1881) Scottish essayist and historian

(Attributed)

Variants include "one less rascal" and "one less scoundrel."

Widely attributed to Carlyle back to the late 19th Century (e.g., 1873, 1876, 1876), during his lifetime. While that should make a misattribution or misquote less likely, thus far I cannot find the above in online searches of his body of work, nor any references to works that would include it.