So the foe is poisoned and dies in "A Poison Tree"?I just don't understand the ending.

Well, maybe.  There are actually two ways to interpret
this poem.  Depending on the interpretation you choose, the answer is either "yes" or
"no."  It all depends on how you define the word "outstretched."  Let's dissect each
interpretation in turn.

The interpretation that you suggest
here is the simplest one.  It is the one that allows the poem to be read like a nursery
rhyme:  a simple poem about nixing anger through communication with a friend and
fostering anger through secrecy with a foe.  This reading of the poem suggests that the
foe who is "outstretched" beneath the tree is actually dead.  The speaker, then, isn't
very commendable, is he?  He revels in the death of an
enemy.

However, if you want to take the word "outstretched"
at its literal meaning, then the foe isn't dead at all.  In this case, it is only the
friendship that is poisoned.  It is the foe that realizes that his friend is really an
enemy.  The issues and the poisons of anger, hypocrisy, and secrecy still remain, ... as
they often do in real life.  Our enemies live
on.

Considering the base nature of the speaker as well as
the obvious connection to the biblical fall of man, I'll give you one guess as to
whether this poem is found within Blake's Songs of Innocence or
Songs of Experience.  (Ha!)