Did the boys return the horse because they were conscience-stricken or because they were afraid?

In the entire story the boys have been portrayed fearless. It was their craze for horses which made them steal one. However, after keeping the horse for a few months, the farmer John Byro visited their house and saw the horse. He, however, did not say anything to the boys rather praised their family for its honesty. This was a point where the boys realized that in order to keep their family tradition of honesty alive they would have to return the horse to its rightful owner.