The deaths of Romeo and Juliet appear needless, as it was mainly the people around them who were fighting and disagreeing about the relationship. Romeo and Juliet mainly kept out of the fighting but their lives were made miserable and unhappy by the people around them who were interfering, and because they were so deeply in love with each other, they refused to be split up and therefore took risks to stay together which eventually led to their deaths. The Prologue at the beginning of the play suggests it was mainly fate that was to blame, "star-cross'd lovers and "death-mark'd love" are examples of this.
Romeo is a young teenager, a member of the Montague household, who unfortunately falls in love with Juliet, a Capulet whose family are bitter enemies of the Montagues. He blames Fate or misfortune for nearly everything that goes wrong; after he kills Tybalt, he then realises the seriousness of his action but prefers to blame the stars rather than himself.
"O, I am fortune's fool".
I think killing Tybalt was the biggest mistake Romeo made as it led to his banishment and it meant he could no longer see his beloved Juliet, without the risk of being caught. I do not think though that you can blame Romeo for killing Tybalt as he was determined to fight Romeo and when he refused, Romeo's closest friend Mercutio stepped in and drew his sword. They fought and Romeo intervenes trying to stop them but this according to the dying Mercutio gave Tybalt the opportunity to kill him, "I was hurt under your arm". Romeo then fought and killed Tybalt. Later when he hears from Balthasar the news that Juliet is dead he says, "Then I defy you stars", meaning he is not going to let Fate dictate...