Act 3, Sc. 3: Why doesn't Hamlet kill Claudius when he sees him alone and undefended?

Hamlet. Now might I do it pat, now he is a-praying.

And now I'll do't. [Draws his sword]

And so he goes to heaven;

And so am I revenged. That would be scanned:

A villain kills my father, and for that

I, his sole son, do this same villain send

To heaven.

Why, this is hire and salary, not revenge.

He took my father grossly, full of bread,

With all his crimes broad blown, as flush as May;

And how his audit stands who knows save heaven?

But in our circumstance and course of thought

'Tis heavy with him. And am I then revenged,

To take him in the purging of his soul,

When he is fit and seasoned for his passage?

No.

Up, sword, and know thou a more horrid hent:

When he is drunk asleep, or in his rage,

Or in th'incestuous pleasure of his bed,

At game a-swearing, or about some act

That has no relish of salvation in't,

Then trip him, that his heels may kick at heaven

And that his soul may be damned and black

As hell, whereto it goes. My mother stays.

This physic but prolongs thy sickly days.