If we are to agree with Prince Escalus's view expressed in the final scene, then Lords Capulet and Montague are to blame for Romeo's and Juliet's deaths, as we see in Prince Escalus's lines:
Where be these enemies? Capulet, Montage,
See what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love! (V.iii.302-04)
In saying this Prince Escalus is asserting that the couple's death served as God's punishment for Lords Capulet and Montague's hatred. Since both Romeo and Juliet were their parents' only joys, and since they died in part because they had to marry in secret, Prince Escalus is also asserting that God killed Lords Capulet and Montague's only joys in life with their love for each other. While Romeo made some rash and fatal decision, and Friar Laurence also made some poor decisions, none of these mistakes would have been an issue or even existed had the two families not been involved in a longstanding feud. Therefore, Prince Escalus is perfectly right to assert that Lords Capulet and Montague are ultimately to blame for the couple's death.
Romeo made the rash decisions of crashing the Capulet's ball and of hastily marrying Juliet in secret; plus, he made the rash and fateful decision to avenge himself on Tybalt for Mercutio's death. However, had the hatred between Lords Capulet and Montague not existed, Romeo's presence at the ball would never have been an issue; the couple would never have had to marry in secret; and, Tybalt would never have challenged Romeo to a duel due to feeling insulted by Romeo's presence at the Capulet ball, resulting in Mercutio's death and Tybalt's own death, plus Romeo's banishment and eventually his own death as well. In addition, Friar Laurence made the rash but well-intentioned decision to marry the couple, which later led to the need to fake Juliet's death. When the plan to fake her death goes awry, it leads to her real death; however, neither of these things would have happened had, again, the hatred between the two families not existed. Therefore, Prince Escalus is perfectly right to place the blame all of the deaths in the play on Lords Capulet and Montague.
Firstly, this sounds as though you are asking someone to write your essay for you. I won't do that, but I will answer your questions. First of all, it is difficult to say who exactly is responsible for the deaths of Romeo and Juliet. Quite literally, they are responsible for their own demises as they took their own lives. However, it can also be argued that their families are responsible for continuing a hateful fued so long that they will not allow passionate love to grow between the two main characters. Because of this longstanding fued, the lovers saw no choice but to end their lives.
If you want to pin it on one person, Lady Capulet could be your scapegoat. Lady Capulet is vengeful, angry and does not pay attention to her daughter, making her a bit selfish. Lady Capulet ignores Juliet's pleas concerning her impending marriage to Paris and seeks to exact revenge on Romeo for killing Tybalt without weighing her daughters' feelings in the matter.