I also think that Juliet shouldn't have deceived and disobeyed her parents. Like Romeo, she was too hasty in rushing into marriage. She was already engaged to marry Paris at the time of the ball, and until then, had been indifferent to her parent’s decision.
Although Lord and Lady Capulet were partly responsible for keeping the feud going, they had authority over their household, and could have stopped the quarrelling if they wanted to. They should have paid more attention to Juliet's wishes and listened to her opinions. They were too hard on her when she refused to marry Paris.
Romeo’s parents Lord and Lady Montague, like the Capulet’s, they were also responsible for keeping the feud going. If they had been more understanding, Romeo would not have felt he had to keep secrets from them.
The Prince could be to blame too; he should have acted earlier to stop the feud between the two families. It was unjust of him to send Romeo away from Verona as a punishment for killing Tybalt when the fight was Tybalt's fault, Romeo acted partially in self defence. Which means that the blame could also be pointed at Tybalt, he started the fight that resulted in Romeo being banished, and he was always causing trouble. He, more than anyone else, kept the hatred alive between the two families.
Juliet’s nurse also helped Romeo and Juliet in their sinful ways; she encouraged Juliet in her romantic ideas. She carried messages, and helped the lovers to meet and get married secretly. She should have let Juliet's parents know what was going on. She advised Juliet to marry Paris, knowing she was already married to Romeo.
Like the nurse, Friar Lawrence also encouraged the relationship, good intentioned as he may have been. He should never have agreed to marry Romeo and Juliet in secret. He gave Juliet the sleeping drug, and he was responsible for getting news of the plan to Romeo - the news never reached him, so Romeo thought Juliet was dead.
Mercutio did also play a minor role in the deaths of Romeo and Juliet, his attitude to quarrels and fighting made the situation worse. He saw the feud as a game and that led to his death and Romeo's banishment.
Finally I think that fate plays a large part in the resultant tragic deaths of the couple, the tragedy was not the fault of any one individual - it happened because of bad luck. It was fate that Romeo and Juliet met and fell in love, and bad luck that the letter which could have saved them didn't reach Romeo in time. Everything was just in the wrong place at the wrong time.
