What does the "keynote scene" of Maggie: A Girl of the Streets suggest is the work's main theme?Stephen Crane's Maggie: A Girl of the Streets

The broad brush portrait of the Bowery is Stephen Crane's
Realism at its best.  In his effort to portray the life in this environment as
unsympathetically as he can, Crane touches the descriptions with humor, even to the
point of farce.  The opening sentence reads,

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 A very little boy stood upon a heap of gravel
for the honor of Rum
Alley.

What could possibly
involve "honor" in the Bowery?  When the readers comes to Chapter II, they perceive the
squalor, the degradation, the depravity of the
Bowery:

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Eventually they entered into a dark region
where, from a careening building, a dozen gruesome doorways gave up loads of babies to
the street and the gutter. A wind of early autumn raised yellow dust from cobbles and
swirled it against an hundred windows. Long streamers of garments fluttered from
fire-escapes. In all unhandy places there were buckets, brooms, rags and bottles. In the
street infants played or fought with other infants or sat stupidly in the way of
vehicles. Formidable women, with uncombed hair and disordered dress, gossiped while
leaning on railings, or screamed in frantic quarrels. Withered persons, in curious
postures of submission to something, sat smoking pipes in obscure corners. A thousand
odors of cooking food came forth to the street. The building quivered and creaked from
the weight of humanity stamping about in its
bowels.

This scene suggests
that the deprived environment in which the man characters live has much to do with their
victimization.   Here is a portrayal of life with no pretense.  Poverty, brutality, and
a lack of realistic prospects are what face the inhabitants of this small, dirty world. 
That it offers no hope or redemption is clear; the ruin of one as pretty as Maggie is
inevitable in this destructive social environment.