Who or what is the antagonist in Zilpha Keatley Snyder's novel for young readers titled The Egypt Game?

The most obvious antagonist in Zilpha Keatley Snyder’s
novel for young readers titled The Egypt Game is the person who
attacks April (nearly strangling her) and who actually succeeds in killing another
neighborhood girl. The identity of the killer is not revealed until close to the end of
the novel, but surely this person qualifies as the most dangerous
antagonist.

Ultimately the killer is revealed to be a
cousin of Mr. Schmitt, who owns a local store.  The cousin works in the store, and, when
we first meet him, he is described as a “stocky redheaded young man with blotchy
freckles” (p. 129). He never assists customers.

After the
man’s criminal activities are discovered, the narrator reports that
there

wasn’t
going to be a real trial because the man was very sick mentally and was to be sent to a
hospital for the criminally insane. . . . The police hadn’t found out about him before
because Mr. Schmitt had always given him an alibi. (p.
193)

Mr. Schmitt’s attempts
to protect his cousin from the law may make Mr. Schmitt himself another antagonist in
the novel (certainly he is usually described in unattractive terms).  One might even try
to make the case that Mr. Schmitt is the one who is ultimately responsible for his
cousin’s crimes, since the cousin himself is insane, whereas Mr. Schmitt is not.  In
fact, later in the book one character even says, “I’ll bet he had a notion that his
cousin was the murderer but he didn’t want to believe it” (p. 199). There is even
speculation that this is why Mr. Schmitt tried to blame the murder on the Professor,
another member of the neighborhood.  If this is true (and Snyder leaves the matter
ambiguous), then Schmitt is indeed a blameworthy person and may be even more of a real
antagonist, ironically, than the actual killer.  Still, it is the cousin who actually
commits the crimes.

In some respects, Snyder's novel
resembles Harper Lee's To Kill a Mockingbird: a mysterious
character (the Professor) observes children play (thus resembling Boo Radley in Lee's
novel); another character (the red-headed cousin) is mentally challenged (like Boo
Radley); and a mysterious assailant is on the loose and is attacking children (as at the
end of Harper's novel). Of course, no child in Harper's novel actually dies, and the
mentally challenged character in Harper's novel is much more fully developed. Likewise,
the person who proves to be the true criminal in Harper's novel is also much more fully
described and characterized.

[2009 Antheneum
edition]