Which characters are most responsible for shaping the play's progression, especially the tragic ending?
In the final scene of the play, we actually see Prince Escalus place all blame for the play's plot progression culminating in tragedy on Lords Capulet and Montague. Capulet and Montague are to blame for their continuing hatred towards each other and persistent feud that leads to many deaths, including Mercutio's, Tybalt's, and finally their own children's deaths. We see Prince Escalus place the blame on Capulet and Montague in his lines,
Where be these enemies? Capulet, Montage[Montague],
See what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love! (V.iii.302-304)
In saying, "See what a scourge is laid upon your hate," Prince Escalus is asserting that God has punished Lords Capulet and Montague for their hatred. Not only that, God has punished them by taking away what they truly loved, their children.
Tybalt is also responsible for the play's progression and tragic end, however, the feud is still ultimately responsible for Tybalt's actions. By nature, Tybalt has a hot-headed, fiery temper. He is quick to judge and quick to act rashly, based on his false judgements. We first see Tybalt act rashly in the first scene. Tybalt sees Benvolio's sword drawn and assumes that he is fighting the servants when in actuality Benvolio is trying to stop them. Tybalt demands of Benvolio, "What, art thou drawn among these heartless hinds?" and begins fighting him (I.i.61). Tybalt's one decision turns the fight into a horrible mob scene. Likewise, Tybalt's decision to challenge Romeo for crashing the Capulet's ball leads to many deaths, including his own and, indirectly, both Romeo's and Juliet's. Although Tybalt's actions are to blame for so many deaths, his actions are fed by the existence of the feud. Therefore, Lords Capulet and Montague, just as the Prince says, are ultimately to blame for how the play progresses, culminating in tragic end.