Douluo: Martial Spirit Blood, Defying Heaven and Changing Fate

A young man living in the 21st century accidentally came to Douluo Continent because he was trying to save the girl he loved who was hit by a train. Based on his familiarity with the plot, how should this young man change his destiny and return to his former world to protect the girl in his heart? The protagonist has no position and kills as he pleases. If he decisively confirms that he is the enemy, he will never be a saint.