Moses receiving law
Yale Center for British Art, Paul Mellon Collection
To Blake the law resulted from the fall of man. It was given as a temporary measure by Urizen to exercise control over desire in the absence of the Eternal Vision. What Murry calls the 'disastrous separation of Desire and Reason' causes true reason to become false reason (Urizen) which holds desire in chains. "But in the symbolic figure of Orc 'desire is not weak enough to be restrained', and does not sink into the shadow of Desire. It is not indeed pure Desire, because of the fatal separation has occurred. But it is desire with enough memory of its former state of grace to be in rebellion against the bondage of false reason."
Quote is from John Middleton Murry's William Blake, Page 86.
Here is some of what Blake wrote about Moses and about the law.
Song of Los, Plate 3, (E 67)
"Moses beheld upon Mount Sinai forms of dark delusion"
The Everlasting Gospel, PAGES 48-52, (E 521)
"Jesus was sitting in Moses Chair
They brought the trembling Woman There
Moses commands she be stoned to Death
What was the sound of Jesus breath
He laid his hand on Moses Law
The Ancient Heavens in Silent Awe
Writ with Curses from Pole to Pole
All away began to roll"
Annotations to Watson, (E 618)
"All Penal Laws court Transgression & therefore are cruelty & Murder
The laws of the Jews were (both ceremonial & real) the
basest & most oppressive of human codes. & being like all other
codes given under pretence of divine command were what Christ
pronouncd them The Abomination that maketh desolate. i.e State
Religion which is the Source of all Cruelty"
America, Plate 8, (E 54)
"The terror answerd: I am Orc, wreath'd round the accursed tree:
The times are ended; shadows pass the morning gins to break;
The fiery joy, that Urizen perverted to ten commands,
What night he led the starry hosts thro' the wide wilderness:
That stony law I stamp to dust: and scatter religion abroad
To the four winds as a torn book, & none shall gather the leaves;
But they shall rot on desart sands, & consume in bottomless deeps;
To make the desarts blossom, & the deeps shrink to their fountains,
And to renew the fiery joy, and burst the stony roof."
Blake is entirely Biblical in his understanding that the law of sin and death, the law of commandments and retribution, is transcended through living in Jesus whom he calls the Imagination.
 There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.
 For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death.