Does Shylock show much reason when he argues that so-called Christians keep slaves and think nothing of it?
The Duke, who is appointed judge in this case, has already decided that Shylock is "inhuman", though he won't say so to his face. Antonio, however, is more than willing to betray his sense of the Jew as an inhuman beast, incapable of either reason or mercy.
Shylock's argument has merit, since Venetian society does allow slave ownership. He has the law on his side, and he is demanding that he be given the same respect as any Christian slave owner. He is arguing that the immorality of his actions are not the court's business, since they are ultimately no different from the actions of Christian slave owners.