This is a very interesting question because the answer depends a lot on how you wish to stage the play and interpret the subtext. Obviously, the simple answer is that Shylock does not treat Antonio very well at all. From our first introduction to him in Act I scene 3 he can be shown to be cruelly manipulative, goading Antonio into accepting the "merry bond" of forfeiting a "pound of flesh" if he is not able to pay back the loan. It is clear from the beginning that Shylock has a deep hatred of Antonio for his identity as a Christian and how he harms Shylock's own business. From the entrance of Antonio it is clear that Shylock wants to plan revenge for the "ancient grudge" he bears him:
How like a fawning publican he looks!
I hate him for he is a Christian,
But more for that in low simplicity
He lends out money gratis and brings down
The rate of usance here with us in Venice.
Shylock seems a character then who is unswerving in his commitment to gaining revenge on Antonio, even rejecting the repeated pleas of characters such as Portia for mercy, insisting on justice and the law--which, ironically, is what he gets, thanks to Portia's manipulation of the law.
However, I like to try to approach Shylock's character with a more understanding glance. Let us not forget that Antonio has treated Shylock abominably--he has spit at his face and insults him, comparing him to the devil. It is clear that Shylock views Antonio as yet another manifestation of how he and his race have been mistreated since the dawn of time. Therefore it is possible to cast Shylock in a far more sympathetic light and portray Antonio as the "baddie" in this play.