Act 3, Scene 1 of Romeo and Juliet teaches us that resolving conflicts with violence results in disaster.
Although Romeo and Tybalt’s families are feuding, the prince has issued a decree banning sword fights. When Tybalt kills Mercutio under Romeo’s arm, he feels responsible. Tybalt returns to fight him and Romeo loses his cool. He goes from condemning sword fights to participating in one.
Away to heaven respective lenity,
And fire-ey'd fury be my conduct now! (Act 3, Scene 1)
Romeo does kill Tybalt, basically in revenge and self-defense simultaneously. Due to the special circumstances, his punishment is banishment and not death. Still, Romeo can no longer be with his family or his love. He considers banishment worse than death.
We all know what happens next. Romeo has to marry Juliet in secret and then flee, and Juliet fakes her death. Romeo arrives in her tomb and thinks she is actually dead, and kills himself. She wakes up and stabs herself with his knife.
The lesson is clear: When you act in haste for revenge, disaster results. If Romeo had stopped to think, it would have saved Juliet's life, Tybalt's, and his own. It also might have ended the feud.