The Bill Payment List report displays amounts depending on the payment account you use. Choosing a credit card for bill payments will show as positive while selecting a bank account will appear as negative on the report.
Therefore, when you make a payment using your bank, it means you have decreased your asset (-). And making a payment using your credit card account will increase your liability (+).
You can also consult an accountant for more details about how payments affect your bank and credit card accounts.