The difference between the suras and the asuras is that the suras know that nothing can happen without the desire of the Supreme Personality of Godhead, whereas the asuras cannot understand the supreme will of the Lord. In this fight, Vrtrasura is actually the sura, whereas Indra is the asura. No one can act independently; rather, everyone acts under the direction of the Supreme Personality of Godhead. Therefore victory and defeat come according to the results of ones karma, and the judgment is given by the Supreme Lord (karmana-daiva-netrena). Since we act under the control of the Supreme according to our karma, no one is independent, from Brahma down to the insignificant ant. Whether we are defeated or victorious, the Supreme Lord is always victorious because everyone acts under His directions.