Never give a sucker an even break
Literal meaning, but usually used for comic effect.
This line is generally associated with W.C.Fields, who made a film of this name in 1943 and said it as an ad-lib in a stage production of the musical Poppy in 1923. It appears this was more than just a dramatic line and expressed Fields' true sentiments. As well as being a comic actor he was a successful and reportedly ruthless businessman.
Eric Partridge, in his Dictionary of Catch Phrases, says the quotation is "more credibly attributed to a remark made by Edward Francis Albee (1857-1930)", although he offers no evidence to support that view.