William Blake is somewhat rare among British poets: he was both a poet and a painter. Indeed, during his lifetime he made ends meet with his talent for drawing, painting, and illustrating. Despite his popularity now (he is considered to be one of the six major male Romantic poets of the early nineteenth century), Blake was relatively unknown during his lifetime, except as a working-class engraver and illustrator. Blake is also unique in that he combined his two artistic talents and produced a series of what he called "Illuminated Books," books that featured his pictures and poetry on the same page! You can read a very brief account of how Blake made these books here, and you can see some pictures of what they look like over at "Best of the Web."
One of Blake's first illuminated books was called Songs of Innocence (it came out in 1789), which was later combined with his Songs of Experience in 1794. The full title of the volume in 1794 was Songs of Innocence and of Experience Showing the Two Contrary States of the Human Soul. Now, as you might have already guessed, the Songs of Innocence tend to be—though aren't always—more innocent. They feature lambs, children singing, and that sort of thing, whereas the Songs of Experience tend to be a bit darker.
"A Poison Tree," as you've probably figured out by now, appears in Songs of Experience. It's a poem about anger, revenge, and death (some of Blake's favorite themes), which contrast markedly with many of the poems in the Songs of Innocence that feature, well, happier trees and more benign themes. In this poem, Blake is really indulging and exploring his darker side, and the darker side of the human condition by extension.
Do you know anyone who really, really annoys you? Somebody who, no matter what they do, always manages to get under your skin? Maybe it's a roommate who refuses to take out the trash or do the dishes, or perhaps it's a sibling who always listens to their music too loud when you're trying to study or insists on taking the front seat of the car all the time.
Have you ever noticed that when you talk to them reasonably about what they're doing that makes you angry, everything becomes much simpler? If you don't say anything, though, your anger just festers and grows, and grows, and makes you more miserable by the minute. There's a good chance that you're familiar with this experience, and it is this experience that William Blake's poem discusses, though in a more gruesome fashion.
Now, we admit, sometimes it's easier just to walk away. But sometimes a confrontation is in order. If we refuse to talk to people about what they're doing that is bugging us, we're the ones who really suffer; we, essentially, "water" and "sun" (in Blake's terms) our anger until it blossoms into a poisonous apple. Granted, growing a poison apple with nothing but hate is a pretty unlikely scenario—well, it's actually impossible—but it's really an extended metaphor for how destructive anger can be. Sure, it won't turn into an apple that will kill your enemy, but it can become something equally destructive.
Just think: there have been many troubled people who have "snapped" and gone on to do something just as destructive (just think of the school shootings alone over the last twenty years or so). In a sense, Blake's poem urges us (you included!) to talk about our anger and frustrations—not just with our friends, but with our enemies as well. Perhaps this can, at the very least, ease our internal trouble and prevent us from harming others.