Where would I find a good quote that shows how Romeo and Juliet have died needlessly? (It must be from prologue or act 5 scene 3.)
The worst part about the deaths of Romeo and Juliet is the fact that it was the only way to "bury their parents' strife" (Prologue.8). The families were so entrenched in their feud, or gang war as we might call it today, that it took the loss of their most precious children to end it. That is needless. People should be able to work out their differences in a civil and diplomatic way but pride kept the two families feuding and no one even knows how or why it started!
One very famous quote from the prologue that depicts how sad the situation is for Romeo and Juliet is "A pair of star-cross'd lovers take their life;/ Whose misadventur'd piteous overthrows/ Doth with their death bury their parents' strife" (Prologue.6-8). A few things are going on here in this section: first, fate has already decided that the kids will die; second, miscommunication brings about mournful consequences; and finally, this is the only way to stop the family feud.
In the final act, after the kids have died and Friar Lawrence is explaining the situation, he says that he saw Paris and Romeo dead, but he was there when Juliet awoke. He invited her to come away with him, but "she, too desperate, would not go with me,/ But, as it seems, did violence on herself" (V.iii.269-270). Here we see that Juliet could have remained alive had she gone with Friar Lawrence and listened to his counsel. Sadly, she decided to needlessly take her life out of desperation.