Sanhedrin 25 - Those Ineligible to be Judges or Witnesses
The following are ineligible to be judges or witnesses: one who rolls dice (a gambler), one who lends on interest, one who bets on pigeon races, and one who does business with the produce of the seventh year. Rabbi Yehudah says, "When are the dice-players ineligible to adjudicate or testify? When they have no trade but this, but if they also have another trade, they are eligible."
Those who put personal gain above the law are suspect of accepting bribes, but what is the problem with gamblers? Rabbi Yehudah supposes that if all he does is gambling, then he is not knowledgeable neither in the law nor in the ways of commerce. Therefore, if he has another trade, he is eligible. The first Sage, however, considers gambling a form of theft: his customers don't really expect to loose, so when the gambler collect his wins, he is a robber.
Art: Michelangelo Caravaggio - The Gamblers