What does this quote from ROMEO AND JULIETTE mean?

Romeo is talking about how his friend Mercutio just got killed, and how he just killed Mercutio's slayer Tybalt. Tybalt, who only not so long ago became family to him because he married Juliet. He's torn over this, and calling out to Juliet, saying her beauty had made his soft towards his new relation Tybalt. He feels guilty because he was the one challenged by Tybalt, and Mercutio answered it for him.