(Balthasar) Then she is well, and nothing can be ill.
Her body sleeps in Capels’ monument, (Capulet's)
And her immortal part with angels lives.
I saw her laid low in her kindred's vault(20)
And presently took post to tell it you.
O, pardon me for bringing these ill news,
Since you did leave it for my office, sir.
Balthasar hs been in Verona long enough to hear the Juliet is "dead" (actually only in a potion-induced coma, as the audience knows) and laid in the Capulet's tomb. Directly after learning this fact Balthasar left Verona and came to Mantua, without speaking to Friar Lawrence about any of this. Therefore Balthasar only knows what everyone in Verona thinks is the truth: that Juliet is dead. The one man in Verona who could have told him otherwise, Friar Lawrence, does not have a chance to tell him that Juliet is indeed alive.
This makes Romeo wild with grief and immediately suicidal.
Well, Juliet, I will lie with thee to-night.
Let's see for means. O mischief, thou art swift
To enter in the thoughts of desperate men!
I do remember an apothecary,
And hereabouts he dwells, which late I noted(40)
In tatt'red weeds, with overwhelming brows,
Culling of simples. Meagre were his looks,
Sharp misery had worn him to the bones;
And in his needy shop a tortoise hung,
An alligator stuff'd, and other skins(45)
Of ill-shaped fishes; and about his shelves
A beggarly account of empty boxes,
Green earthen pots, bladders, and musty seeds,
Remnants of packthread, and old cakes of roses
Were thinly scattered, to make up a show.(50)
Noting this penury, to myself I said,
‘An if a man did need a poison now
Whose sale is present death in Mantua,
Here lives a caitiff wretch would sell it him.’
Romeo remembers that there is a very poor apothecary's (chemist or pharmacist) shop here in Mantua. In that shop were all manner of stuffed or preserved poisonous animals, and, Romeo reasons, if it is at all possible to buy poison (which is illegal, by the way) in Mantua, this is the one place to buy it. Romeo sends Balthasar to hire horses for them to ride back to Verona, and Romeo plans to go and procure the poison for himself. He plans to go to Juliet's tomb in Verona, and lay himself in the tomb next to her and take the poison, thus killing himself. Romeo does all of these things, not knowing that Juliet is still alive.