What two poems would be well suited to compare with "A Poison Tree"?
In addition to the above poems mentioned, you might try "Home Burial" by Robert Frost. It, too, deals with a relationship but also deals with the nurturing of resentments, wrongs, etc. In the poem a wife nurses and nurtures long-held, perceived shortcomings on the part of her husband, until eventually a crisis results.
The Medieval ballad, "Twa Corbies," might also be relevant if you're looking for poems about relationships that aren't what one thinks, although I'm not sure that that's what you're looking for.
If you want perceived wrongs and dark or off-beat revenge, Browning's "My Last Ducchess" would certainly qualify.
I would suggest that you compare “Poison Tree,” by Williams Blake with “The Arrow and the Song” by William Wadsworth Longfellow. Both of these poems address our actions with our acquaintances or friends and the repercussions of those actions. I would also suggest you read and compare the poem, “Are You Digging on MY Grave,” by Thomas Hardy. All of these poems are about relationships and how we deal with them. The Hardy poem poses this reflection from the grave of an individual who has passed away and how the relationships s/he thought were meaningful might not have been.