This poem is a tricky one, and it is pretty hard to understand. Blake is basically commenting on the contradictions that exist in the world, and how life and its mysteries often baffle people and leave questions unanswered. He ties it to humans, and how human beings are often the source of these confusions, contradictions and incompatible events that occur.
In stanza one, Blake states that when people-for whatever reason, on purpose or not-make others poor, that makes pity possible. Poor people often elicit our pity. Also, "if all were happy as we," then human beings would not be capable of feeling mercy. Mercy is often felt because the person feeling it has experience pain and suffering before, so can relate to someone else feeling it, and gives them mercy.
The second stanza he implies that peace exists not because people are kind, but because "mutual fears" on the opposite sides of the issue motivate a truce. One side is afraid the other will do something horrible, and vice-versa, so they declare peace so that their fears won't come true. Then, he states that peace stays in place until "selfish loves increase"; people get more selfish, and want things, so then "cruelty" comes into the picture and "knits a snare" for people to get caught in.
In the third stanza, he continues with the cruelty personification, saying that once cruelty comes into play, it settles in, and "waters the ground with tears". But then, because cruelty exists in the world, and people are being beat down, "humility takes its root underneath its feet". In stanza four, enter the concept of mystery, and how it spreads everywhere, baffling all because we don't know the answers to certain things, and "the fruit of deceipt" then comes into play. So all of these traits have created a tree; cruelty the seed, humility the root, mystery the shade the tree groes in, and deceipt is its frut that "the raven" eats while it sits in the tree. A very confusing mix of traits, indeed.
The last stanza says that all of the things that happen on earth, all of the ways that we humans behave, even confuse "the gods of the earth and sea". They look and look to see where this mangled tree came from. But, "their search was all in vain" because the place this tree originated from was "the human Brain."
Basically, humans are confusing creatures, who let their vices spawn good and bad results, that are all connected on one great whole. I hope that explanation helps; good luck!