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. Examples of Trapped by Gambling Debts include: