Dear Otokonoko,

The following website explains it
beautifully:

http://homepages.tesco.net/~eandcthomp/macbeth.htm

"Macbeth, King James, and the Witches."

Thanks for the suggestions. I guess it has always puzzled me because it is so succinct (where else in Shakespeare can you find a statement so short) and seems almost benign as the other witches have been running ships aground and conjuring storms, and so on. I suspect that this is one of the many cases when reading too much into the script can be a bad idea. The line just seems to stick out and jar with me.

I know that later in the script is a scene involving the witches, which is generally believed to have been by someone other than Shakespeare. I forget which it is, but once it is pointed out to you, it is quite obviously different, the verse is of a lower quality, and is quite insipid and jingly. This means that we do not have the original script, and it is possible that other parts of the play, including witches' parts, were incompletely transmitted to us, or erroneous. That first scene does seem to be all Shakespeare's; however perhaps that particular line was originally misquoted?

Yes, at secondary school we were told that someone else had written the witches' scene in, just to add a light touch to all the gore and psychosis of the murders. Wasn't it the scene with the cauldron bubbling, toil and trouble and all that?
Since a lot of Shakespeare was satirical, killing swine might have referred to those to whom the pearls were cast, but that is probably too subtle in this context. As Cornflower says, it's probably best taken at face value.
I suspect that Wm Shakespeare was the pen name of a syndicate, meaning Freelance. It might just as easily have been Willy Wagstaff if they had found someone with that name to be the front man.
I like Brenda James's idea that 'he' was (or included) Henry Neville the English ambassador.

Otokonoko

Have A look here to see what the usual diet of peasants was in Shakespeare's time and I think you will agree with him that a witch killing a pig 'just because' was a shameful act of violence and waste.

http://www.foodtimeline.org/foodfaq3.html#peasants

The language is simple and blunt because the playwright aimed that bit directly at the many peasants and others in the 'front' rows. You will notice that Shakespeare's language and images change often subtly to suit the character and in this case a large section of the audience. The wanton destruction of a pig says a lot to those on a subsistence diet, who rarely see meat on the table and are at the mercy of the elements (and their betters).

Understand it as a clever line because that is what it is. Listen to skilled politicians whose rhetoric is studded with little gems everywhere to appeal to various sections and interests of the audience.