Although Tybalt is a villain, he is still a cousin of Juliet's, and after their marriage, he is then related to Romeo as well. Besides being officially "family," there had recently been made a law in Verona that no one is to fight between the two families. Anyone who is caught fighting will be punished by death. The Prince announced that, and all of them knew the consequences. When Romeo is banished, the two lovers cannot be together. This is what begins the confusion. They no longer can communicate and rely on others to get messages back and forth. That is what causes both deaths to occur in the end. Had Tybalt lived, Romeo never would have been banished, and they would have been able to announce their love/marriage to their families.
While Tybalt is portrayed as a villain, his death is not portrayed as a positive event because of its impact on Romeo. Just after killing Tybalt, in Act III, Scene I, for example, Romeo is banished from the city of Verona by the Prince. His banishment may be preferable to the sentence of death but, in being exiled, Romeo does not have the opportunity to make public his secret marriage to Juliet. Instead, the hatred between the Montagues and Capulets only intensifies.
In addition, because of Tybalt's death, Lord Capulet arranges for Juliet to marry Paris because he thinks that she needs a "sudden day of joy" to end her sadness (as a result of Tybalt's death). In reality, however, Juliet's sadness is not just about Tybalt; she weeps for Romeo's banishment, too. Had this unwanted marriage to Paris not been arranged, Juliet may have never taken the sleeping potion.