The speaker of the poem describes Shakespeare as "wicked" in the first line of the third stanza. That stanza reads,
Surely, Shakespeare is wicked, the map a bad example.
With ships and sun and love tempting them to steal—
For lives that slyly turn in their cramped holes
From fog to endless night? On their slag heap, these children
Wear skins peeped through by bones and spectacles of steel
With mended glass, like bottle bits on stones.
All of their time and space are foggy slum.
So blot their maps with slums as big as doom.
We also hear a reference to Shakespeare in the second stanza; here are the relevant lines:
Cloudless at dawn, civilized dome riding all cities.
Belled, flowery, Tyrolese valley. Open-handed map
Awarding the world its world. And yet, for these
Children, these windows, not this map, their world,
Where all their future's painted with a fog,
A narrow street sealed in with a lead sky
Far far from rivers, capes, and stars of words.
Given the imagery
surrounding the mentions of Shakespeare, we can infer what makes Shakespeare a "wicked" figure for this poem's speaker. Shakespeare writes about worlds that are "cloudless at dawn," where "ships and sun and love" are common occurrences. The speaker worries that the children's knowing about the existence of these things will "tempt them to steal" in order to escape the "cramped holes" and "endless night" that define their world. For the speaker, showing the children all the things they don't have is cruel; ships sailing across seas on a map are irrelevant to children who live in "slums as big as doom" and are starving, as evidenced by their wearing "skins peeped through by bones." In essence, Shakespeare dangles before these children that which they cannot have. It would make more sense to decorate the classroom with maps that more realistically display the conditions of the slums so as not to tease the children with the impossible.
All is not lost in this poem, though; the speaker does appeal to the government and to us (the reader) to work so that these maps could maybe become a possibility for these children. Rather than limiting their lives to the "catacombs" of the slums, the poem argues in the fourth stanza, we should fix the education system so that school will "show the children to green fields, and make their world / Run azure on gold sands."