According to the evidence presented in the play, Shylock's hatred toward Antonio is based on the fact that Antonio is a Christian who treats him poorly and lends money without charging people interest. In Act One, Scene 3, Bassanio assures Shylock that Antonio will pay back his loan of three thousand ducats. When Antonio enters the scene, Shylock says to himself,
"I hate him for he is a Christian,
But more for that in low simplicity
He lends out money gratis and brings down
The rate of usance here with us in Venice.
If I can catch him once upon the hip,
I will feed fat the ancient grudge I bear him.
He hates our sacred nation, and he rails,
Even there where merchants most do congregate,
On me, my bargains and my well-won thrift, Which he calls 'interest.' Cursèd be my tribe
If I forgive him!" (1.3.34-44).
Essentially, Shylock's cruelty toward Antonio is not because he is jealous of Bassanio's relationship with him, but stems from the way Antonio has treated him in the past, as well as Antonio's business practices. Shylock proceeds to remind Antonio that Antonio has called him a "misbeliever" and a "cutthroat dog." Shylock also recalls a time when Antonio spat on his clothes and the numerous moments Antonio has publicly insulted him. Overall, Shylock's cruelty is his response to Antonio's antagonistic behavior toward him, which fuels his revenge.