How is Shakespeare's scene where Clarence dies both frightful and amusing in Richard III?

In Act I, scene four of Shakespeare's Richard
III, Richard has outwardly arranged to pardon his brother Clarence (who he
secretly took steps to have arrested and put in the Tower of London), while he has
(again) secretly made other arrangements—to have Clarence
murdered.

The scene is frightful—but rather than amusing,
personally I find it disturbingly ironic. The irony comes from the substance of a dream
Clarence (George Plantagenet, 1st Duke of Clarence, 1st Earl of Salisbury, 1st Earl of
Warwick) has had that he shares with Brakenbury. Clarence describes having escaped from
the Tower (in his dream) and is sailing to href="http://en.wikipedia.org/wiki/Burgundy_wine">Burgundy. While this is a
place in France, it is also the name of a kind of wine. Wine will become an important
detail in the "horror" of scene four, so it is also not just a play on
words, but foreshadowing as well. (Shakespeare was a clever
and artful writer.) Clarence goes on to describe sailing with his
brother Gloster. They look (across the English Channel, we assume) toward England. "Wars
of York and Lancaster" is a reference to the War of the Roses—the battle between the two
royal houses for supremacy in England—and, of course, the
throne.

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Clarence is filled with guilt for having switched
sides more than once during the Wars of the
Roses…

Gloster and Clarence
(still in the dream) recall the "thousand heavy times" they had experienced during the
war. Suddenly, as Gloster and Clarence walk on the "giddy" (unsafe; weak) hatches on the
deck, Gloster falls into the water to his death, and Clarence has a flash of insight in
the dream (foreshadowing, again) of what it would be like to
drown.

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Methought that Gloster stumbled; and, in
falling,
Struck me, that thought to stay him, overboard
Into the
tumbling billows of the main.
O Lord, methought what pain it was to
drown!
What dreadful noise of waters in my ears!
What sights of ugly
death within my eyes!

Even
though Clarence tries to reason with the murderers that he is a king's son, and it is a
sin to kill him, they will not listen. Clarence, even in death, cannot believe Richard's
betrayal. When the murderers kill Clarence, they "drown" his body in a "vat of Malmsey
wine."

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FIRST
MURDERER.

[Stabs
him.]

Take that, and that: if all this will not
do,

I'll drown you in the malmsey-butt
within.

[Exit with the
body.]

With the
"drowning" of Clarence's body in the wine, the irony is complete, in that his fearful
dream regarding his brother's drowning actually foreshadows his
own death—murdered but then placed in a "vat" or "barrel" of
wine.

Additional
Source:

http://en.wikipedia.org/wiki/George_Plantagenet,_1st_Duke_of_Clarence