Why did Song Wu kill the tiger?

Song Wu is a hero in the novel Water Margin in the Ming Dynasty. He is famous for fighting tigers. The story is set in the late Ming Dynasty. When Song Wu went back to his hometown to visit his brother Wu Da, he learned that Wu Da was bullied by Ximen Qing and decided to avenge his brother. Because Ximen Qing was too cunning to get justice through legal means, Song Wu decided to resort to murder. With the help of his friends and Liang Shanbo's friends, he planned a drunken pretending to hang himself and killed Ximen Qing. Later, Song Wu was hired by the imperial court, appointed as a police officer and became a representative of the chivalrous.

As for the story of killing tigers, it happened in the early stage of the story. According to the plot of the novel, when Song Wu was killing pigs in Jiangzhou, he came to a bridge and found someone whispering on the bridge. It turned out that the traces left by the tiger's stealing and killing were seen by the local county magistrate, and people were sent to catch the tiger, but in vain. So, the confused officials decided to behead all the people responsible for catching tigers. Song Wu decided to go out after learning about it. When hunting tigers, Song Wu fell down on the way, shot through the tiger with an arrow to stop it, and then hacked it to death with a pen at close range. At this point, Song Wu became a hero and was praised by everyone.