In the play The Merchant of Venice, what does Shylock mean when he says "My deeds upon my head"?

In Act four, Scene 1, Portia is attempting to settle the dispute between Shylock and Antonio. After Shylock admits to the bond, Portia tells Shylock that he must be merciful. She demands that Shylock shows mercy on Antonio in the same manner that God shows mercy on mankind. Shylock then says, "My deeds upon my head!" (Shakespeare 4.1.204). Shylock's comment essentially translates to him saying that he will answer for his own behavior. Shylock is telling Portia that he will take responsibility for his own decisions. He does not care that he will possibly face damnation for not showing mercy to others and disobeying the law of God. Portia then asks Shylock if he can settle with Antonio, and Bassanio explains to her that he already offered Shylock twice the bond. However, Shylock remains obstinate and refuses to settle, claiming that all he wants is his bond.