What does Romeo say is worse than poison?
In a word: money.
This line is found in Act 5, Scene 1, Lines 79-85 (5.1.79-85).
This line is Romeo's, speaking to the apothecary (druggist). He wants to commit suicide by purchasing poison. His friend Bathlazar had just informed him of seeing Juliet dead in the Capulet tomb.
It is against the law to sell poison in Mantua, but Romeo, jaded as he is, knows the druggist will sell him what he wants for a few ducats. Morality tends to fade when money is to be made.
The apothecary does sell him the poison. Romeo takes it, but his attitude is that it is he who has sold the apothecary poison, for money destroys men's souls. As for Romeo, he wants to exit this mortal life with his soul intact.
Here is the full context of the lines to the apothecary:
There is thy gold, worse poison to men's souls,
Doing more murders in this loathsome world,
Than these poor compounds that thou mayst not sell.
I sell thee poison; thou hast sold me none.
Farewell: buy food, and get thyself in flesh.
Come, cordial and not poison, go with me
To Juliet's grave; for there must I use thee.