When analyzing a theme in literature, it's easier to look at the evidence of the theme you are thinking of using in order to come up with your arguments. From there, you can then combine your arguments in order to generate your thesis. This forces you to look at the evidence first. I notice that if students do it the other way around (thesis then arguments then evidence), they sometimes get attached to their thesis and try to make evidence fit that just doesn't.
Anyway, since you have your thesis already, let's break it down. It should read something along the lines of this: "In the play Romeo and Juliet, William Shakespeare demonstrates the immense damage a feud can cause through the rivalry between the Montagues and the Capulets."
The meat of your thesis statement is the idea of immense damage. This, I believe, is where you will find your arguments. Like I mentioned before, it's sometimes easier to look at evidence first. With that being said, here are some examples (there are many more) of immense damage that the feud caused from the play:
- In Act I, Scene I, the feud causes turmoil in Verona, and the Prince, who is fed up with the fighting, makes his decree
- In Act I, Scene 5, Tybalt recognizes Romeo at the party, and asks someone to fetch his sword; luckily, Capulet is able to cool him down
- In Act II, Scene 3, the Friar basically agrees to marry Romeo and Juliet in an attempt to end the feud (in a sense, the feud forces Romeo and Juliet to marry in secret in the first place)
- In Act III, Scene I, Tybalt kills Mercutio, who is neither a relative of the Montagues nor the Capulets (following this, Romeo is exiled from Verona)
- In Act V, Scene III, Montague and Capulet join hands, ending the feud since both of their children have died
Within the evidence, there is a pattern of death and deceit that snowball into even greater consequences.
Considering the evidence, some arguments that can be made are:
- The feud causes the death of innocent civilians (Mercutio, Paris)
- The feud causes senseless fighting that disturbs the peace (fighting in Act I, death of Mercutio, fighting between Tybalt and Romeo)
- The feud causes people to be deceitful (Romeo and Juliet's relationship, the Friar keeping it a secret)
- The feud causes people to make rash decisions (Romeo and Juliet marrying, the Friar's intentions with their marriage)