"A Poison Tree" is a poem about anger, and, more importantly, some of the destructive consequences that can result when we cultivate our anger, rather than try a more productive outlet for this potentially dangerous emotion (like stamp collecting!). The fact that Blake refers to anger as "wrath" suggests that the poem is about a more serious type of anger, a vengeful or spiteful feeling of biblical magnitude ("wrath" is commonly used in the Bible to refer to the anger of Old Testament God). In other words, this is seriously powerful stuff that must be carefully guarded against.
While Blake's poem illustrates some of the destructive consequences of anger, it also suggests that it is a normal emotion that needs to be dealt with properly. Talk it out, folks.
"A Poison Tree" tells us that anger is never just anger by itself. It needs fear and sorrow to develop. Worst recipe ever.