How did the secretive marriage of Romeo and Juliet cause the death of others?
Since Tybalt is unaware that Romeo and Juliet are married, he does not consider Romeo a kinsman. Upon coming across Romeo in the street, Tybalt challenges Romeo to a duel. He does so because he previously witnessed Romeo and Juliet kiss at the Capulet ball, and he feels insulted that Romeo would 1) sneak into the ball despite the feud between their families and 2) kiss Juliet. Had Tybalt realized the extent of their love, his anger may have been mollified. Tybalt remains unaware of their marriage, however, and though Romeo does not accept the challenge, Mercutio does. Tybalt kills Mercutio, and Romeo kills Tybalt.
Later in the play, Paris also dies as a result of their secret marriage. After Juliet takes the sleeping potion, Paris believes she is dead. While mourning at her grave, Paris observes Romeo, who also believes Juliet is dead, and Paris erroneously assumes Romeo has come to the grave to vandalize it. Romeo and Paris battle. Romeo kills Paris, then commits suicide.