Blake's message, in my view, is that revenge is not justified, though he doesn't state this directly, and the poem describes only an isolated case without background or elaboration regarding this particular quest for retribution. The more important point, however, may be that when a person's anger is unexpressed, it leads to consequences that are destructive to that person as well as to the object of his or her "wrath."
It's significant that nothing is said about the reasons for his anger, in the cases of both his "friend" and his "foe." On the surface, the poem is morally neutral. At the end, the speaker is happy with the result: his poisoned tree has accomplished its purpose. But few readers would conclude that Blake's own view is the same as the speaker's. The underlying point is that the mayhem could have been avoided if he had spoken openly about his feelings instead of hiding them and planning a secret revenge. In other words, as common wisdom from our own time over 200 years later has it, if you don't talk about problems, they never get resolved.
Blake understood the dark side of human nature. Taken out of context, his attitude could be judged as merely cynical in this and other poems and in striking observations such as
Pity would be no more
If we did not make somebody poor,
And mercy no longer would be
If all were as happy as we.
Yet the fact that he views this aspect of human behavior with such clarity and penetration is evidence that he understands that there is something wrong with it. A person who believes that revenge is justified would presumably not be writing poems showing death from a poisoned apple as the result. The speaker's vengeance is revealed as something underhanded and cowardly. It's also hypocritical, and it stems from weakness in the human spirit instead of strength. He
waters it in fears
Night and morning with [his] tears.
He uses "deceitful wiles" against the enemy. Despite his congratulating himself at the final moment, his whole process of revenge is portrayed in such negative terms that few readers would feel any empathy for him. The tone of vindication is ironic, because the man carrying out his plan of death merely condemns himself in our eyes rather than justifying his act.