Fate is the cause of Romeo and Juliet's death. No one is to blame because everything happens for a reason. It was fate that they fell in love and happened to be on rival sides of the family. It was fate that the plaque spread when the messenger was trying to deliver the letter to Romeo. It was also fate that they fell in love at first sight. They didn't let the opinions of their family members affect them and they went with how they felt. It was destiny for them to be together even if it meant dying along each others side.
But everyone has a different opinion, this is mine. Some may even say it wasn't fate.