Why does Don John pretend that he does not recognize Claudio?

A major theme in Much Ado About
Nothing is "appearance versus reality," and there is no better setting and
occasion to develop this theme than a masquerade where everyone is masked.  Everyone, it
seems, is on equal status.  Otherwise unmasked, Don John (a bastard) would have no
clout, since his status is so low.

In this scene Don John
is appearing to be honest.  That's irony number one.

Irony
number two is that he knows Claudio is Claudio, but he pretends to address him as
Benedick:

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Are not you Signior
Benedick?

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Signior, you are very
near my brother in his love:
he is enamoured on Hero; I pray you,
dissuade him
from her: she is no equal for his birth: you
may do the part of an honest man in
it.

With Don John
gone, Claudio removes his mask and says:

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Thus answer I in the name of
Benedick,
But hear these ill news with the ears of
Claudio.

Irony
number three is: why does Claudio answer in the name of Benedick?  Is he jealous of
Pedro, or does he really not trust Hero, or both?  As most men were distrustful of women
(a glaring double standard), maybe Claudio wants to hear gossip about her just to give
him a reason to dump her.  As a vice figure, Don John knows all
this.

So, Don John is pretending to not recognize Claudio
because he wants to bait him with lies about his brother Don Pedro wooing Hero for
himself in order to create animosity between Claudio and Pedro.  Instead of playing
matchmaker, Don John, out of spite, is trying to be a
breaker-upper.

He wants to hurt both Claudio and Pedro by
using a female's reputation against their own.  So, if Hero is of low reputation, then
no one will have her for a wife and everyone will be unhappy, just like him.  Misery
loves company.

You see, a bastard (see below) will do
anything to create strife, even if there's no goal or advantage gained.  Don John's evil
comes from having no father and, thus, no status in a society that only looks favorably
upon sons with fathers.  Name, status, reputation were the only things that mattered to
men at this time.

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The BASTARD: the dispossessed son, he burns with
resentment. He can’t have what he wants, so he lashes out to hurt those around him. His
deeds are often for effect – he wants to provoke action in others. He proudly announces
his rebellious dealings. Don’t be fooled by his boyish demeanor – he’s a bundle of
hate.