It is mostly Capulets' and Montagues' fault that Romeo and Juliet died. If they didn't hate each other so much, they would let their children marry, and they would be able to live happily ever after

Romeo and Juliet first meet each other at a party at the Capulets' house. They fall in love with each other at the first sight without knowing that they are from rival families, and when they do learn about it, it is too late. Juliet says: "My only love, sprung from my only hate! Too early seen unknown, and known too late!" Juliet realizes that it was a mistake to fall in love with Romeo, because he is one of the people she should watch out for. He is her first and only love, but he is an enemy of her family. Luckily for Romeo, Juliet is smart, and such dumb thing as unreasonable hate between their parents wouldn't stop her from loving Romeo and getting married with him. She is sorry that their families are enemies, though, because everything would be much easier if they weren't. So this is the first time when the hate between Capulets and Montagues becomes an obstacle to Romeo and Juliet's happiness.

Only after their children die, do the Montague and Capulet come to their senses. The Prince accuses them of killing Romeo and Juliet: "Capulet, Montague, See what a scourge is laid upon your hate, That heavens finds means to kill your joys with love!" The Prince makes them see how wrong they were all that time, that all because of their hate, because they couldn't let their children love each other, Romeo and Juliet died. After that, Capulet and Montague make peace, but it is too late, nothing they do now will ever bring their children back.

On the other hand Romeo & Juliet are responsible for their own death. If they didn't kept their romance a secret and told their parents how much they loved each other, their parent might understand them and let them marry. They were too afraid that their parents wouldn't understand their love.

It is mostly Capulets' and Montagues' fault that Romeo and Juliet died. If they didn't hate each other so much, they would let their children marry, and they would be able to live happily ever after. There was only one person that realized how unreasonable the fighting was and even tried to stop it, and he was Benvolio. Benvolio was a Montague, but the Capulets didn't have a peacemaker such as Benvolio. They only had Tybalt, whose only goal was to destroy all the Montagues. Capulet and Montague learned about their children's love when they were already dead. It is still mostly Capulets' and Montagues' fault that Romeo and Juliet died, but maybe it was just their fate to die.