Act 2, Sc. 2, lines 557-594: What reason or excuse does Hamlet give himself for putting on the play before taking his revenge?

Hamlet. And can say nothing - no, not for a king,

Upon whose property and most dear life

A damned defeat was made. Am I a coward?

Who calls me villain, breaks my pate across,

Plucks off my beard and blows it in my face,

Tweaks me by the nose, gives me the lie i'th'throat

As deep as to the lungs? Who does me this?

Ha!

'Swounds, I should take it; for it cannot be

But I am pigeon-livered and lack gall

To make oppression bitter, or ere this

I should ha' fatted all the region kites

With this slave's offal. Bloody, bawdy villain!

Why, what an ass am I! This is most brave,

That I, the son of a dear father murdered,

Prompted to my revenge by heaven and hell,

Must like a whore unpack my heart with words

And fall a-cursing like a very drab,

A scullion! Fie upon't! Foh!

About, my brains. I have heard

That guilty creatures sitting at a play

Have, by the very cunning of the scene,

Been struck so to the soul that presently

They have proclaimed their malefactions.

For murder, though it have no tongue, will speak

With most miraculous organ. I'll have these players

Play something like the murder of my father

Before mine uncle. I'll observe his looks;

I'll tent him to the quick. If he but blench,

I know my course. The spirit that I have seen

May be a devil, and the devil hath power

T'assume a pleasing shape, yea, and perhaps,

Out of my weakness and my melancholy,

As he is very potent with such spirits,

Abuses me to damn me. I'll have grounds

More relative than this. The play's the thing

Wherein I'll catch the conscience of the King.