Although Romeo and Juliet made many mistakes, which led to many consequences, the grave mistake was made, when Romeo could not suppress his emotions over the death of Mercutio, his best friend. As we all know, that Tybalt caused Mercutio's death because Mercutio had provoked him, into duelling. Even though they were just fooling around, Mercutio had gotten slain in the process unintentionally by the tip of Tybalt's blade. Romeo, who had witnessed the slaying of Mercutio, simply could not suppress his rising anger, and in retaliation killed Tybalt this was a big mistake in Romeo's part.

Throughout the process of Mercutio getting slain by Tybalt, and Romeo slaying Tybalt, the prince was alerted and had come to see what had happened. Eventually, everything got sorted out but not for the best of things, this is because Romeo ends up getting banished from Verona as punishment for the...