A gambler in debt doesn't have a complete fun after winning, because he'd feel he'd have to refund the won money back to the person he's owing. Along the line, he'd be getting different thoughts on what to do with the money, whether to gamble some more or just clear his debt. Paying back leaves the player broke again, hence he'd definitely gamble again to recover enough money to clear his debt and have remaining money. If the outcome doesn't equate the amount the gambler is looking to gain, he's most likely going to borrow some money again to meet his goals, while in debt. All the trouble he'd go through, can make the player depressed in the process, as he wouldn't understand what's actually happening with him. Most people who gambled with borrowed money didn't earn enough winnings to repay their debt.
Gamblers who are addicted won't really be contented when they have already won. It will just make them crave for more because they aren't satisfied with one time winning and is aiming for a consecutive win regardless how risky it may be. Additionally, if they have a debt to pay, it will make them think that the winning prize they have isn't enough. It will just become a small amount after they have paid what they just borrowed so it will make them chase winnings to fulfill the amount they really desire and to cover their other expenses. They won't care much whether they will stake more. Which will make their funds at risk. So instead of taking home some winnings, some even ended up having nothing or additional debt on their name.
I always ask myself why people aren't stopping after they won a decent amount in gambling. To a certain extent I can understand that a win might only feel like a small win and it would be nice to turn it into a bigger win instead. But we always face the risk of losing our win and worst case maybe even turn that into a loss. Especially when borrowed money is involved the stakes are much higher and any loss will double has hard, because we not only have a lower bankroll but also have to repay the loan we took out. Which is why I wouldn't use any borrowed money for gambling. The risk is just too high and it's not going to end well in many cases.