Shakespeare warns us of upcoming events in the First Prologue, even the deaths of Romeo and Juliet, in order to lay out one of his most central themes in the play, fate vs. free will. In the very beginning of the play, even in the prologue, Shakespeare's writing poses the question: Did Romeo and Juliet die merely because they were destined to die, or did choices and actions lead to their deaths?
Throughout the course of the play, Shakespeare seems to be arguing that their deaths were caused by a little bit of both--both fate and free choices--but mostly by choices. He even lays out this answer in the very third line of the prologue, "...[f]rom ancient grudge break to new mutiny" (3). This line points to two significant facts. First, the family feud between the Montagues and Capulets was an "ancient grudge," meaning a longstanding feud, possibly even going back generations and generations. But secondly, and more importantly, the present Lords Capulet and Montague have broken "to new mutiny," meaning that, while peace had been established between the two families, the present lords have decided to rehash the argument again, unearthing "ancient" grudges and even getting the whole city of Verona involved in the violence. Therefore, this third line makes it very evident that Lords Capulet and Montague's choices play a central role in the drama and are, possibly, ultimately responsible for the play's outcome.
Shakespeare's point about fate is begun in the fifth line, "From forth the fatal loins of these two foes / A pair of star-cross'd lovers take their life" (5-6). It is due to fate that both Romeo and Juliet were born to the parents of these two warring families; however, that seems to be the greatest role that fate plays. Romeo is ultimately characterized as a very classic tragic hero, a virtuous character of high social standing who has some sort of character flaw that leads to his demise. Romeo makes many choices that lead to his own death, such as crashing the Capulet's ball and avenging himself on Tybalt for Mercutio's death. However, despite the fact that Romeo is a tragic hero, Shakespeare also makes a point of placing the ultimate blame for all of the deaths in play on Lords Capulet and Montague, as we see in Prince Escalus's important lines, "See what a scourge is laid upon your hate, / That heaven finds means to kill your joys with love!" (V.iii.302-04).
Hence, Shakespeare relays the upcoming deaths in his prologue in order to lay out his theme of fate vs. free will and, also, to begin to hint at his answer for which was more responsible, fate or free will.