In the death scenes of both Tybalt and Mercutio, the central conflict centers around defending the family's honor. Tybalt challenges Romeo in the first place because he feels that Romeo's attendance at the Capulet party was an affront to the Capulet family honor. When Romeo does not meet the challenge, and instead attempts to be civil to Tybalt, he is likewise showing honor to his new wife.
Mercutio is defending the Montague clan when he steps into the fray. In fact, one could say that he is defending the Montagues in place of Romeo, as Romeo appears to be capitulating to Tybalt (keep in mind that neither Tybalt nor Mercutio know that Romeo has recently married Juliet). After Mercutio dies, Romeo defends the honor of his friend by seeking to avenge the death. Thus, honor and loyalty lie at the heart of the scene and are at the crux of the conflict therein.