In Act II, scene iii, Friar is speaking of the power of plants when he notes that the plant has several powers. Plants can be medicinal, and they can be poisonous. As he finishes he uses a metaphor that foreshadows the death of lovers as they are products of kings:
Two such opposed kings encamp them still
In man as well as herbs, grace and rude will;
And where the worser is predominant,
Full soon the canker death eats up that plant.
Later, in scene iv, Mercutio and Benvolio are talking about Romeo when Mercutio notes that Romeo's love for Rosaline is killing him. Little does Mercutio know that Romoe's love is not for Rosaline, but for Juliet, and that her love will literally kill him:
Alas poor Romeo! he is already dead; stabbed with a
white wench's black eye; shot through the ear with a
love-song; the very pin of his heart cleft with the
blind bow-boy's butt-shaft
Mercutio uses language figuratively here as hyperbole or exaggeration, but the foreshadow is clear. Love will kill Romeo.