An interesting question. I'd say Shakespeare does this both through language used by Shylock and language used by others to describe him. When Shylock is talking about eating with Christians (Antonio), he refers to eating pork as being where the demons were cast. When he discusses Antonio, he flatly states his hatred, and plans to take a "pound of flesh." That's a lot of flesh, and if taken near the heart, would make him a murderer. He repeats this desire, and calls for vengeance even over money in Act III.
As far as what is said about him, the way Antonio talks about him in Act I makes him sound evil, and in Act III Antonio flatly states " He seeks my life."