How does the following quote show that Shylock is a villain?To bait fish withal. If it will feed nothing else, it will feed my revenge..... Act 3, scence 1 line 52
In order to see how this shows that he is a villain, just think of what Shylock is saying in this scene. Think about how nasty and vengeful it makes him look.
In this line, he is answering Salerio. Salerio is asking him, basically, why he doesn't just take money instead of demanding the pound of flesh that Antonio owes him. Salerio asks him what he's going to do with a pound of flesh anyway.
At this point, Shylock speaks the lines you cite. Basically, he's saying "I don't care what I do with it. I'll use it for bait if I want. The whole point is that I'm going to get revenge.
So what does that say about Shylock? It says that (at least in this instance) he is a nasty and vengeful man. He'll risk killing Antonio to get his revenge even if the revenge will mean that he gets no tangible benefit from the deal (like money).