What is the overall effect of the mood changes in Shakespeare's Romeo and Juliet, Act 3?

In Romeo and Juliet, by William
Shakespeare, the things that most dramatically change the mood of the play
are:

Tybalt picks a fight with Mercutio and while Romeo
tries to break up the fight, Tybalt slips under Romeo's arm and kills Mercutio. Romeo
has done everything he possible to placate Tybalt, refusing to fight with him (as they
are now related). However, once Mercutio dies, Romeo goes looking for Tybalt, and when
they fight, Romeo kills the other man.

So Romeo is first
guilty of killing a member of the enemy family of the Capulets. But he is also guilty of
killing one of his wife's family members. And this is something Juliet will for a short
time struggle with.

However, when the incident comes before
Prince Escalus, he does not order Romeo's death, under the circumstances, but he
banishes Romeo, who may never return to Verona again. Romeo believes that he might as
well be dead—as being separated from Juliet means
death.

Ha!
Banishment! Be merciful, say 'death'. / For exile hate more terror in his look, / Much
more than death. Do not say banishment. (III, iii,
13-15)

Romeo and Juliet have
their one night of wedded bliss; too soon, Romeo must leave or be killed for still
residing in Verona.

If all of this is not enough, Juliet's
parents insist that Juliet marry Paris within the week, and threaten to disown her if
she does not. Then the Nurse tells Juliet to forget Romeo and marry Paris, even though
Juliet is already married. At this point, Juliet turns her back on the Nurse, no longer
confiding in her.

It is during Act III that things begin to
unravel. It will be because of these occurrences and circumstances that Juliet will take
steps to avoid marriage to Paris, and the misadventures of our star-crossed lovers will
lead to their tragic end.