Shylock is presented to us as a devious, vengeful man, embittered by years of anti-Semitic abuse and persecution to the point where he's become hell-bent on avenging himself against Christians. He claims he only wants equality, to be treated the same way as Christians. But in actual fact, what he really wants is the same kind of right to avenge himself on Christians as they have in relation to Jews.
Shylock is stubborn and inflexible, insisting that his bloody bond be carried out to the absolute letter. Such an attitude is entirely in keeping with the prevailing anti-Semitic stereotype of Jews as narrowly legalistic in the practice of their religion, privileging law above spirit. In terms of his personality, Shylock's refusal to yield an inch on the terms of his bargain makes him look vindictive and spiteful. No longer is his bond concerned with justice, but rather with revenge, and that's a whole different thing altogether.