If what you allege your ex-husband is true, he has clearly violated the laws of Allah. First of all, he owes you your mahr, unless you have asked him to divorce you, without any fault of his own (or in other words, unless he has abused you physically or emotionally or failed to provide for you, and hence you were forced to ask for a divorce). If not, then he owes you the mahr he promised you at the time of marriage. If, however, the divorce took place before consummation of marriage, he owes you only half of what was promised; otherwise he owes you the full mahr. Allah says:
"And if you divorce them before having touched them, but after having settled a dower upon them, then [give them] half of what you have settled - unless it be that they forgo their claim or he in whose hand is the marriage-tie forgoes his claim [to half of the dower]: and to forgo what is due to you is more in accord with God-consciousness. And forget not [that you are to act with] grace towards one another: verily, God sees all that you do." (Qur'an: 2:237).
As for the debts, he has incurred through the credit cards, he is responsible to pay them. If he fails to clear them, he is guilty in the sight of Allah. The issue of debt is indeed a serious matter. You can gauge the seriousness of it only when you consider the fact that the Prophet (peace be upon him) refused to pray janazah on those who died with unpaid debts--unless he was able to pay them on their behalf. Therefore, your ex-husband is clearly in breach of the laws of Allah.
If you don't forgive him, he will be among those who in the words of the Prophet are the most unfortunate ones of his ummah. The Prophet once asked his companions, "Do you know who is the bankrupt one?'; they said, "he is the one who has no money or wealth', he said, "but the bankrupt person of my ummah is the one who appears on the day of judgment before Allah, with prayers, fasts, and charities to his credit, while, at the same time, he has been guilty of insulting people, making false allegations against them, devouring their wealth unlawfully, shedding the blood of the innocent, striking them, etc. So the rewards of his works will be distributed among his victims. If they become exhausted before his debts have been paid off, their sins will be imposed on him; hence he will be flung into the hell-fire." (Reported by Muslim on the authority of Abu Hurayrah).
I pray to Allah to help us to appear before Him while being free of the bondage of debts, injustice, or violating the rights of humans or fellow creatures.