Which characters helped cause Romeo and Juliet to commit suicide?

Though Romeo bought his own poison and Juliet stabbed herself, in a sense, many are responsible for their deaths. Juliet’s cousin Tybalt and Romeo’s friend Mercutio set off a chain of events that destroys the young lovers. In spite of the prince’s ruling against fighting, Tybalt seeks to quarrel with Romeo. Mercutio immediately offers to duel with Tybalt, who quickly takes him up on it:

TYBALT: Gentlemen, good e’en: a word with one of you.

MERCUTIO: And but one word with one of us? couple it with something; make it a word and a blow.

TYBALT: You shall find me apt enough to that, sir, an you will give me occasion.

Benvolio begs them to stop, but they brawl until Tybalt kills Mercutio when Romeo attempts to break them up. In a rage, Romeo then fights and kills Tybalt and is banished, which leads to his and Juliet’s eventual suicides.

At the play’s conclusion, the prince blames himself and the couple’s parents:

See, what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love.
And I for winking at your discords too
Have lost a brace of kinsmen: all are punish'd.

Lord Capulet and Lord Montague have kept up the feud that kept Romeo and Juliet apart and that perpetuated violence in Verona’s streets. The prince believes that he has been negligent. His punishment is losing Paris and Mercutio, two of his relatives. Note that the prince says, “all are punish’d.” He later says, “Some shall be pardon'd, and some punished,” meaning the city’s ruler is holding many accountable for this tragedy.