Shylock lists a number of insults that Antonio has heaped upon him over time. He cites Antonio's disapproval of his business practices, Antonio's name calling (misbeliever, cut throat dog), and the fact that Antonio has spit upon him publicly.
When called to account, Antonio responds that he will likely do it again.
I am as like to call thee so again,
To spet on thee again, to spurn thee too.
If thou wilt lend this money, lend it not
As to thy friends, for when did friendship take
A breed for barren metal of his friend?
But lend it rather to thine enemy,
Who, if he break, thou mayst with better face
Exact the penalty.