How is Shylock presented as bloodthirsty and selfish?
Throughout the history of the play, and definitely from the perspective of modern audiences, Shylock is one of Shakespeare's most complex characters. He has been portrayed as a ridiculous money-grubbing villain, a man rebelling against his oppressors, the epitome of evil, and more. Some of his words and actions absolutely suggest someone who is interested in violence to a distasteful degree. The most relevant example of this is his relentless pursuing of his "bond," the pound of flesh Antonio is meant to give him if he is unable to repay the loan. Upon hearing that Antonio will not be able to repay the loan, Shylock says:
"I'll plague him [Antonio];
I'll torture him" (3.1.115-116).
Shylock's glee at a consequence that won't get him his money back and may kill Antonio is shocking to the Christian characters and doesn't buy him a lot of sympathy with the audience either, especially as he is completely unwilling to back down, despite all arguments and protestations until he is forced to.
Additionally, one could look to his daughter Jessica and her flight from her father and his house as evidence of his cruelty. In Act II, scene 3, she claims her house is "a hell" and is more than happy to run away with her father's money and mother's ring and live it up with the Christians. When she does this, Shylock's reaction is also telling, running down the street yelling about his "daughter" and "ducats" (money) in equal amounts.
Still, there are two sides to these examples. Antonio, for instance, has made no secret of his anti-Semitism and has regularly abused Shylock, not to mention made business hard for him. It's not surprising that Shylock would harbor such deep hatred for him. Furthermore, Jessica is shown to be frivolous in her selling of her mother's ring to buy a monkey, of all things. Though Shylock has a blood-thirsty element in his personality, he also has some reasons for his actions.