A truth that's told with bad intent Beats all the Lies you can invent.
Suddenly, Blake snaps out of metaphor mode. This couplet is more of a proverb—a catchy little saying, a maxim, an aphorism.
"A truth that's told with bad intent" is worse than any kind of lie, because some lies can be well-intentioned, possibly even helpful. There's such a thing as a "white lie," in Blake's view. But if someone's using a truth for an evil purpose—that's obviously bad, not only because of its effect, but because it makes the truth seem corrupted, too. You can use a lie to help the truth, and you can use a truth to damage the truth. It's sort of a paradox.
What would be an example? Maybe stomping all over someone's dream—like saying, "You'll never get to be a professional ping pong champion!" It might be true—but what's the point, aside from hurting someone's feelings?
It is right it should be so; Man was made for Joy & Woe; And when this we rightly know Thro' the World we safely go.
Don't expect perfect happiness, Shmoopers. Then again, don't expect total misery. Expect a mixture of both, says Blake, and you'll have the right perspective on life. You'll be able to get through it without feeling utterly cheated.
Also, joy and suffering seem to exist as a complementary pair—you can't have one without the other.
Note that he's still using couplets, but these four lines are all part of the same argument—unlike any of the couplets that have come before, except for the first four lines of the poem (the opening "grain of sand" part).
He's also still in proverb mode like in the last couplet—he's not referencing any critters.
The fact that he keeps the same rhyme through all four lines demonstrates that these are four lines that hang together—they're not like the couplets from before.