Romeo and Juliet meets at a party not knowing that their families are enemies, once they realize, its too late and they are already in love and decide they want to get married. Despite Friar Laurence attempt to settle the Capulets and Montague feud it didn't help when the family found out about their secret marriage.
Tybalt Capulet is also the main villain in the play, he did evil things like killing Romeo friend Mercutio which made everything
worse when he faced Mercutio in a sword fight. After that Romeo wanted to kill Tybalt and he succeeded which led him to begin exiled by the prince. Tybalt wanted to hurt romeo when he first saw him at the capulets party.
After Juliet believes Romeo is dead, she decides to drink potion and kill herself because she cant live with the pain. But the potion that she drinks was part of friar Laurence's plan to appear dead so Romeo could retrieve her when she woke up and Romeo and Juliet could escape to Verona.
In conclusion, the family feud was the biggest scenario to blame for the death of Romeo and Juliet. If Tybalt would've never killed Mercutio Romeo wouldn't be sent into exile. Also if Friar john could've found Romeo and gave him the letter before Juliet went into the tomb the plan would've worked out perfect.