The Problem with A Poison Tree

Of all the poems in the songs of Experience I thought the Poison Tree was a good take on explaining how hatred and envy can destroy someone. I thought it was very odd  though how in this set of poems with such a clear message of morality would have a poem about destroying another person, even if that person is someone you don't like and killing them without it being admonished or shown in a light of horror or disgust. The only reason I can think of to do this is that it's a allegory that the foe is killed but the narrator is just as "dead" as their foe because they had to "feed" the seed of hatred with tears of fear and deceit a clear showing of that hatred and wrath change a person yet their are no ultimate consequences for the narrator. The religious symbols of a snake and a apple are clear references to Lucifer and the ultimate sin of pride and envy that got him and a third of the host kicked out of heaven, so perhaps the narrator is lucifer with the fow being humanity, a very disturbing idea.