So the evil guys blackmail them into whatever they need in exchange for having the debts paid. They have been gambling too much and now have to pay or face serious consequences, ranging from Knee-capping to actual death, especially if they owe money to an Illegal Gambling Den. The easiest one is that they need lots of quick money.
But you don't want them to do it For the Evulz, so they need a motivation. You need a character to betray your hero, or kidnap someone, or be The Mole.
Booker DeWitt's Mysterious Employers, BioShock Infinite
