I think what a lot of people aren't commenting on here is that the way Shylock is written changes over the course of the play. His big sympathetic speeches all happen in Act 1. In Act 2 Portia gets all the famous rhetoric ('the quality of mercy is not strained' etc) and Shylock degenerates into an entirely one-note villain. It's a reasonably common phenomenon in Shakespeare, where the ending is ostensibly comedic - all the young couples end up together, all the loose ends are tied up, the overall mood is happy - but one character, who we do still feel some sympathy for, doesn't fit into the picture & gets tossed aside. It's the same with Malvolio in Twelfth Night, for example. The revised ending where Shylock converts to Christianity, I think, is meant to write him back into the picture and alleviate the sense of guilt we feel - but of course, for a modern audience, his conversion arguably feels even more uncomfortable.