This exclamation breaks the iambic pentameter (or is an unmetered line of its own). It is found only in the First Folio version of the play, and thought by some editors to be an actor’s addition. The Arden Shakespeare notes:

Simon Russell Beale’s gentle Hamlet could not bring himself to say it…[editor-critic Philip] Edwards, however, argues that it is the turning-point of the speech.

If Edwards’s reading is correct, “vengeance” is one of the words that causes Hamlet to distrust his own speech–perhaps he can’t say it convincingly.

Watch Richard Burton, Kenneth Branagh, and David Tennant’s interpretations:

https://youtu.be/uxV1SgCwruI?t=2m8s

https://youtu.be/QH5_E0MnLj4?t=2m

https://youtu.be/dyB4ktn7AIE?t=3m2s

Arden Shakespeare notes:

bawdy: “lewd, immoral (see bawdry at 438)”

*kindless: “lacking natural feeling” (see “a little more than kin and less than kind”).

This cascade of frustrated insults prompts Hamlet’s reflections on the ineffectuality of “words.”

The speaker continues to refer to the Captain as a father figure. The Captain’s head rests against the speaker’s arm, but he can’t feel anything because he has died. His lack of “pulse” and “will” may suggest the political willpower that is dying with him.

The speaker’s failure to revive the Captain contrasts with section 40 of “Song of Myself,” in which Whitman portrays himself as an almost Christ-like healer of the dying:

I seize the descending man and raise him with resistless will,
O despairer, here is my neck,
By God, you shall not go down! hang your whole weight upon me.

[….]

Not doubt—not decease shall dare to lay finger upon you;
I have embraced you, and henceforth possess you to myself;
And when you rise in the morning you will find what I tell you is so.

“Song of Myself” appeared in 1855; Whitman went on to volunteer as a nurse during the Civil War. His experiences in army hospitals may have sobered his vision of himself as healer, while influencing lines like this one.

In The Masks of Hamlet (1992, pp. 452), Marvin Rosenberg suggests that these lines should be spoken slowly, with pauses, as the idea unfolds: “the words do not come easily.” He further notes that:

The germ of Hamlet’s idea serves a multiple Shakespearean purpose. It supplements his defense, in the ongoing controversy about the stage, of the moral function of actors and the theatre, by referring to a known confession….It reaches out and embraces the spectators….And it gives Hamlet a spring forward into action. He will do something….

(But that “something” is very different from what the Ghost commanded him to do.)

If he so much as turns pale, I’ll know what to do.

blench: blanch, turn pale. Marvin Rosenberg calls this “a splendid knot of a word, often hurled out” by actors as they deliver the speech.

malefactions: crimes, misdeeds.

A crucial transition in the soliloquy. Hamlet turns from the impulse toward immediate and direct revenge to a much more indirect plan: stage a “scene” that will sting Claudius’s conscience and get a revealing reaction out of him. This may humiliate the king, may help confirm his guilt (as well as the Ghost’s honesty, as indicated below)–but it won’t kill the king or enact the revenge Hamlet has promised to take.

For an account of how different actors have handled this transition throughout history, see Marvin Rosenberg’s The Masks of Hamlet (1992, pp. 451-52). Rosenberg notes that “guilty creatures sitting at a play” is a meta-theatrical (fourth wall-breaking) moment that “reaches out and embraces the spectators.”

pate: head.

Plucks off my beard: pulling or plucking a man’s beard was a form of mocking challenge. Compare Claudius in 4.7: “let our beard be shook with danger.”

lie i' the throat: “To lie in one’s throat was proverbial…Hamlet intensifies it” [Arden Shakespeare].

Beating himself up for his inaction, Hamlet imagines a series of humiliations or deliberate provocations by an enemy–then says he is (or must be) so cowardly that he’d “take it.”

The Arden Shakepeare (Third Series, p. 276) notes that “Am I a coward?” and these subsequent rhetorical questions “have sometimes provoked responses from the audience, notably in the case of David Warner’s 1965 performance.”

David Warner as Hamlet (via BBC)

Now that the truth is out about their mission, Rosencrantz and Guildenstern appear (but perhaps only appear) to be back in Hamlet’s good graces.

Good my lord: “a parting formula = farewell” [Arden Shakespeare].

God be: some editions have “God buy,” i.e., goodbye. The critic Dover Wilson suggested Hamlet might be speaking here “with sarcastic relief after [R & G] have gone.”

A friendly reminder to the Player not to get himself in trouble by behaving as Hamlet himself does? The Arden Shakespeare (Third Series, p. 274) speculates:

Perhaps Hamlet is regretting his own behaviour and discouraging the other Players from imitating him. Or perhaps he is warning them not to compete with him in this respect.

ha’t: have it, i.e., have it performed.

See note above.