There are several deaths in Romeo and Juliet, Mercutio is killed by Tybalt and then Romeo kills Tybalt. These deaths contribute to the ongoing feud between the houses of Montague and Capulet. Mercutio is of the Montague house and Tybalt is of the Capulet house. Of course, both Romeo and Juliet both die, as a result of their miscommunicated plan.
Friar Lawrence's message to Romeo never arrives, therefore, Romeo believes that his love, his wife, Juliet is dead, and he kills himself by drinking poison, to join her. When Juliet awakens from her death-like sleep, she sees that Romeo is dead and tries to drink the rest of the poison, but kills herself with his dagger.
There are four deaths all together, each triggering an event that contributes to the ultimate tragedy, the death of the two young lovers.
Friar Lawrence is not arrested, no one is arrested, the Prince attributes the deaths to the families inability to get along.
"After the bodies of Romeo and Juliet are discovered, the friar offers the prince a summary of what has happened. Having confirmed the story with Romeo's letter to his father (delivered by Balthasar) the prince absolves the friar of wrongdoing, calling him a "holy man" (V.iii.270), and blames the feuding families for the deaths of Romeo and Juliet."
Mercutio is stabbed by Tybalt, and Tybalt is stabbed by Romeo; both die. Romeo and Juliet both die from suicide at the Capulet's tomb. Paris dies fighting with Romeo at the tomb. Lady Montague dies from heart failure at the thought of Romeo's exile.
No one is arrested at the end of the play. The Capulets and Montagues make peace since their children have died due to a "star-crossed" love. Friar Lawrence is not arrested and is not implicated in any of the deaths.