Where and why does the tone shift in Hamlet's soliloquy in Act Two, scene two, of Shakespeare's Hamlet?

In Hamlet's soliloquy in Act Two, scene two, of
Shakespeare's Hamlet, the Prince of Denmark berates himself for not
having taken action against Claudius, for Old Hamlet's murder, long before
now.

O, what
a rogue and peasant slave am I!
Is it not monstrous that this player here,

But in a fiction, in a dream of passion,(545)
Could force his soul
so to his own conceit
That from her working all his visage wann'd,

Tears in his eyes, distraction in's aspect,
A broken voice, and
his whole function suiting
With forms to his conceit? And all for
nothing!

Hamlet calls himself
a rogue and a slave: unable to act even though it is in his heart to do so. Only in an
environment of illusion, a play, can he see an expression of emotion that he wants
himself to make. His reference to Hecuba is to the wife of Priam, in the story of the
Trojan War, where her husband Priam is killed by Pyrrhus, a reference Hamlet will soon
make when speaking to one of the visiting players (actors) at the castle. Hecuba stood
by and did nothing when her husband was killed.

In the
play, the actors play their parts, but they are only parts: they mean nothing. And
Hamlet looks to himself also as one who is playing a part: he know of his father's
murder, yet he does not know how to move forward and act on it. (His tragic flaw is
indecision.) However, he also wants to make sure that he is justified in killing a king,
which would have been a grave concern to his Elizabethan
audience.

Again, Hamlet faults
himself:

Yet
I,
A dull and muddy-spirited rascal, shrink,
Like a dreamy fellow,
not full of my cause,
And can’t say anything, no, not for a king
On
whose kingdom and most dear life
Were all brought to nothing. Am I a
coward?

He lacks the impetus,
he says: unable to act even for a dear king (his father) whose life and kingdom were
stolen him. He wonders if he is a coward.

However, Hamlet
renews his purpose to avenge his father's death as he promised the
Ghost:

Fie
upon't! Foh!
About, my brain! Hum, I have heard
That guilty
creatures, sitting at a play,
Have by the very cunning of the scene(585)

Been struck so to the soul that presently
They have proclaim'd
their malefactions;
For murder, though it have no tongue, will speak

With most miraculous
organ...

Hamlet decides he
will delay no longer. He will search for proof. He has heard that when people see in a
play something that is akin to a guilty action they have committed, they give themselves
away with an expression of their guilt. It is in this way that Hamlet changes
the course of his actions—noted in the change of his tone in the speech—to
find proof of Claudius' guilt and proceed to punish him, thereby avenging his father's
murder.

The
play's the thing
Wherein I'll catch the conscience of the
King.

Hamlet will use the
play to move the King's conscience.