How is Romeo affected by Balthasar’s news?

In act 5, scene 1 of Romeo and Juliet, Balthasar brings his master, Romeo, the terrible news of Juliet's death. Juliet isn't actually dead, but after taking Friar Lawrence's potion that's put her into a deep sleep, it appears to everyone that she is indeed dead. In any case, Romeo is utterly devastated at the news, as we might expect—but he's also defiant. He rails against the bad fortune that has brought about what he thinks is the death of his beloved:

Then I defy you, stars!

Defiance soon turns to resolve as Romeo determines that he will head back to Verona that very night, kill himself, and lie next to Juliet. He says he knows of a shabby old pharmacist in Mantua who'll be able to make him a batch of deadly poison. Wasting no time, Romeo heads off to see the pharmacist and demands that he sell him the most lethal poison he has. The pharmacist is reluctant to do so, as he'll be breaking the law. But he's also incredibly poor, and Romeo's money is too tempting to turn down.