What kindness does Shylock show? Why?

Shylock and Antonio are about to go to a lawyer and seal their deal when Shylock suggests a further condition to their agreement: if Antonio doesn't pay back the money he borrows from Shylock, then Shylock will insist on taking a pound of his flesh. Shylock calls this a "kindness" because he won't insist on seizing all of Antonio's goods, as would normally be the case when someone defaults on a debt.
But this condition is very far from what most people would regard as an act of kindness, not least because it will most certainly result in Antonio's death. Although Shylock refers to this stipulation as "a merry sport" (i.e. as nothing more than a joke), he's actually deadly serious about enforcing this blood bond should Antonio default on the debt. The reason is that Shylock wants to gain revenge on a Christian for all the prejudice, hatred, and anti-Semitic abuse he's suffered all these years.