The wanton Boy that kills the Fly Shall feel the Spider's enmity.
This is pure poetic justice—karma, the law of cause and effect. What you do comes back to get you.
In this case, the word "wanton" means that the boy is cruel in a totally unprovoked way. He's just ripping the wings off flies for recreation, to pass the time after school. The kid is like some sort of young Jack the Ripper in training. But he's going to get his.
Blake is toying with the idea that the boy, as punishment for torturing flies, will be reincarnated as a fly and get killed by a spider. Blake doesn't necessarily think that this, specifically, is going to happen. But he does think that wrathful and cruel people are bound to fall victim to wrath and cruelty: the universe is going to fling it back at them.
He who torments the Chafer's sprite Weaves a Bower in endless Night.
A "chafer" is a bug—a scarab beetle, specifically. Since Blake thinks that even bugs and worms have God in them, someone who tortures a bug seems kind of awful, in his eyes. He's covered the bases with cruelty to birds and mammals—now he's going down into the insect world. This couplet, the one before it, and the two after it all deal with bugs in some way.
Blake says it's wrong to torment the chafer's "sprite." This does not mean that this person would be torturing a refreshing, carbonated beverage in a green plastic bottle, belonging to a certain scarab beetle. A "sprite" is a legendary creature—an elf-like being—but it comes from the Latin word for spirit ("spiritus"). So, Blake means that you shouldn't torment the soul or spirit of the beetle, since that's the part that belongs to and is part of God.
By committing acts of cruelty toward the souls of bugs, a person is weaving "a Bower in endless Night." A "bower" is a shelter made from tree branches and vines woven together—so, cruel actions create a home in "endless night," a world of hellish loneliness. Cruelty seals you in with your own egotism, since acting cruelly demonstrates that you don't care about anyone else.
Of course, the night might be endless, but not the time you'll spend in it. Blake says that the souls of living things are what's eternal, not the time they spend in different states of being. Hell, for him, is more of a state of mind—if you find animal cruelty pretty entertaining, you're probably already in hell, as far as Blake's concerned. He leaves the possibility of making a 180 and becoming more compassionate totally wide open.