Is the borrower's failure to repay a debt guaranteed by the deceased considered a sin and a debt upon the deceased's conscience, and are his heirs obligated to repay that debt?

The death of the guarantor does not invalidate the guarantee. If the guarantor dies before the debt is due, the debt becomes immediately payable and is taken from his estate. The heirs are obliged to settle the debt, and then they can claim it back from the guaranteed party when the debt's due date arrives. However, if the guarantor dies when the debt is due or after, and the guaranteed party is present and solvent, the debt is not taken from the guarantor's estate, unless the creditor stipulated the right to demand payment from either of them. Guarantee means that the guarantor's liability is engaged for the debt of the person he guaranteed. His liability is not discharged except by the termination of the guarantee due to the termination of the principal's obligation (by performance of the debt, absolution, or set-off), or by the termination of the guarantee itself (by reconciliation, invalidation of the contract, annulment of the contract, fulfillment of a condition for discharge, or expiration of the temporary guarantee period).