Trapped by Gambling Debts

You need a character to betray your hero, or kidnap someone, or be The Mole. But you don't want him (this trope rarely applies to women) to do it For the Evulz, so he needs a motivation. The easiest one is that he needs lots of quick money. And why would he?

This trope is the answer. He has been gambling too much and now has to pay or face serious consequences, ranging from Kneecapping to actual death. So the evil guys blackmail him into whatever they need in exchange for having his debts paid. That does it: He's not evil, just desperate.

It's an easy move, because you have a character with a dangerous flaw and a clear motivation, but who still can be good if the story needs it. Kind of a Discredited Trope maybe because of that, specially in spy stories, but still used occasionally.

Often happens to The Gambling Addict, especially after an Absurdly High Stakes Game. May be the victim of the Professional Gambler or a crooked Bookie. Compare Wager Slave.

Examples of Trapped by Gambling Debts include:

Anime and Manga

Kaiji is constantly trying to find a way to pay off his debt. It usually ends up with him in worse debt.
The male protagonist of Hayate the Combat Butler is trapped by his parents' debts.
Subverted by Tsunade in Naruto, who uses her jutsu to change her looks from an old woman to a young girl and everything in between, and is always on the move with Shizune, to escape her creditors, with IOU note in tow. (This aspect of her character may be inspired by the Tanuki.)

Comic Books

In one of the origins of The Joker, he was a poor schlub who needed to pay off his gambling debts so he got roped into being the Red Hood (the alleged head of a gang but really just a guy the rest of the gang hires to be The Face of the Gang), then fell in a vat of chemicals and became the Joker.
- Henchmen that actually consider working for The Joker (known for offing henchmen for any or no reason) are generally very desperate for money, often due to this trope.
In Tintin, this is how Colonel Boris/Jorgen trapped Frank Wolff into becoming The Mole.

Film

The plot of Lock, Stock and Two Smoking Barrels is driven by the main character's need to pay off a massive gambling debt owed to the gangster Hatchet Harry. Harry's hoping to use the debt to force the guy's father to hand over his pub.
In the movie Dirty Work, the gambling-addicted Dr. Farthing (Chevy Chase) will only raise Pops on the heart transplant list if he is paid $50,000 to save him from his bookie.
In Lucky Number Slevin, the Boss and The Rabbi rope Slevin into their war by mistaking him for Nick Fisher, a lowlife who owes them tens of thousands of dollars. Of course, Slevin planned for the mobsters to mistake him for Fisher.
In A Knight's Tale, William bails Geoffrey Chaucer out of his gambling debts, and in return, Chaucer travels as Will's herald.
On the second to last play of The Replacements, Nigel confesses to Shane that he has to throw the game in order to not lose his bar as a result of gambling debts. Shane decides to not handoff the snap and improvise instead.

Literature

24 Hours in the Life of a Woman has a man ready to comit suicide because he lost all his money. The title woman saves him, gives him money for a fresh new start... and he wastes it gambling again.
Black Library novel Hammers of Ulric has Anspach the Templar and his debts to the head of the criminal organization in Middenheim.
In the Lord Darcy novel Too Many Magicians, uncovering one character's gambling debts and the associated blackmail forms a major subplot.
Early in the Garrett P.I. series, Morley Dotes gets into trouble because of his addiction to water-spider races.
Ludovic Bagman in Harry Potter and the Goblet of Fire.

Live-Action TV

In at least one episode of the Poirot series of Agatha Christie adaptations starring David Suchet, a character was persuaded to get involved in a jewel robbery to pay off a gambling debt.
One of Garibaldi's men is compromised this way in an early episode of Babylon 5.
Veronica Mars has Jacky's father who is blackmailed into working security for a morally ambiguous casino owner in order to pay off his debt.

Newspaper Comics

A series of Get Fuzzy comic strips had Bucky facing the threat of getting killed by another cat, because Bucky lost a bet on a baseball game and didn't have enough money to pay the debt. The team that Bucky bet on was the Mariners, but he lost that bet because one of their players had retired before he even made the bet.

Radio

In the first Harry Nile episode, Harry owes a gambling debt to a mob boss and is sent to kill a guy to pay it back.

Tabletop Games

In Magic: The Gathering, Liliana Vess made a Deal with the Devil with four archdemons in order to gain eternal youth. Being the selfish woman she is, she decides to try to get out of it by killing them. She's one-fourth of the way there.
Murphy's World. In the adventure "Robyn's Summer Romance in Asgard", one way given to railroad the PCs into the scenario is force them to gamble and lose big, thus requiring them to go on the adventure to pay off their gambling debt.

Video Games

In the Ace Attorney series, Glen Elg fits this trope.
In the second Knights of the Old Republic, a Twi'lek laborer is so addicted to Pazaak that he gambled away his girlfriend'.' You can play the dealer to whom he owes the debt in order to bail out the girlfriend. Either she dumps her idiot boyfriend, or you order her to go with her idiot boyfriend, or you can take ownership of her yourself and keep her wages while telling the idiot boyfriend he's out of luck.
Roman Bellic's gambling problems are why his cousin Niko ends up getting in trouble at the start of of the game.

Real Life

Truth in Television as far as the Mafia were/are concerned. A favourite way for them to get their hooks into a business was to let a gambling-addicted owner run up impossible debts with a mob bookie.
One of the reasons Marc Anthony was so loyal to Gaius Julius Caesar was that the latter covered the former's gambling and prostitute debts that in modern terms stacked up to about US$5 million.