It is not easy to understand Blake's attitude toward Good & Evil. Northrup Frye, a renowned student of Blake has given some explanations in Fearful Symmetry. Let's go to him and to Mr. Blake himself for some help.
Northrup Frey, Fearful Symmetry, Page 197
"Moral good and moral evil do not represent any genuine opposition. The one wages wars and executes criminals; the other murders. The one exploits labor; the other robs. The one exploits marriage on the destruction of virginity; the other rapes. But they have a common enemy, the power of genius and prophecy. In terms of moral good it is not the murderer or the robber but the prophet who is really evil. Barabbas can be safely released, for it is impossible that his robberies can destroy the social structure of Pilate and Caiphas; but there is deadly danger in Jesus and John the Baptist, who must be got rid of at all costs."
Northrup Frey, Fearful Symmetry, Page 56
"Hence evil is a negative: all evil consists either in self-restraint or restraint of others. There can be no such thing, strictly speaking, as an evil act; all acts are good, and evil comes when activity is perverted into the frustration of activity, in oneself or others...
This death-impulse, this perverted wish to cut down and restrict the scope of life, is the touchstone not only of all the obvious vices, but of many acts often not classified as such, like teasing, instilling fear or discouragement, or exacting unthinking obedience."
William Blake, Annotations to Lavater (E 601)
"But as I understand Vice it is a Negative...Accident is the omission of act in self & the hindering of act in another, This is Vice but all Act [<from Individual propensity>] is Virtue. To hinder another is not an act it is the contrary it is a restraint on action both in ourselves & in the person hinderd. for he who hinders another omits his own duty. at the time
Murder is Hindering Another
Theft is Hindering Another
Backbiting. Undermining C[i]rcumventing & whatever is Negative is Vice."
Northrup Frey, Fearful Symmetry, Page 58
"But self-development leads us into a higher state of integration with a larger imaginative unit which is ultimately God. Hence the paradox that one gains his life by losing it, which Jesus taught. The selfish or egocentric are incapable of developing themselves; that comes from expansion outward not withdrawal inward. Hence there are two selves in man absolutely opposed to one another, the better self that grows and lives and the worse self that rots and withers, the good and evil angel:"
William Blake, Annotation to Lavater (E 594)
"Man is a twofold being. one part capable of evil & the other
capable of good that which is capable of good is not also
capable of evil. but that which is capable of evil is also
capable of good. this aphorism seems to consider man as
simple & yet capable of evil. now both evil & good cannot
exist in a simple being. for thus 2 contraries would. spring
from one essence which is impossible. but if man is
considerd as only evil. & god only good. how then is
regeneration effected which turns the evil to good. by
casting out the evil. by the good.
See Matthew XII. Ch. 26. 27. 28. 29 vs"
["If it is Satan who is expelling Satan, then he is divided against himself - so how do you suppose that his kingdom can continue? And if I expel devils because I am an ally of Beelzebub, what alliance do your sons make when they do the same thing? They can settle that question for you! But if I am expelling devils by the Spirit of God, then the kingdom of God has swept over you unawares! How do you suppose anyone could get into a strong man's house and steal his property unless he first tied up the strong man? But if he did that, he could ransack his whole house."]
Northrup Frey, Fearful Symmetry, Page 58
"Man has within him the principle of life and the principle of death: one is the imagination, the other the natural man.
The only possible cure for the original sin of this Selfhood of the natural man is vision, the revelation that this world is fallen and therefore not ultimate."
I think this leads us right back to the first quote; which says in essence that the enemy of the moral system is the prophet who can destroy the status quo.
Los and his Spectre
Jerusalem, Plate 6