When we are first introduced to Launcelot in Act II scene 2, he is still the servant of Shylock, but in his soliloquy he resolves to leave his master and look for better employment, and therefore becomes the servant of Bassanio later on in the play. This soliloquy is revealing because in it Launcelot discusses the pros and cons of leaving his master's employment. In spite of his conscience, which tells him to stay with the Jew, he considers that his employer, Shylock, is "the very devil incarnation" which prompts him to ignore his conscience and leave his master's employment.
Interestingly though we have little direct evidence from the text that Shylock mistreated Launcelot badly. Launcelot does protest that he was mistreated under Shylock:
I am famished in his service; you may tell every finger I have with my ribs.
And yet, it appears that Launcelot is a character prone to exaggeration. In Act II scene 5, we can say that Shylock is rude to Launcelot, but not insulting, and certainly we can explain his asperity with Launcelot by the fact that Launcelot has just left his employment.
Thus, depending on how the director plays Launcelot, you can portray Shylock as a tyrannical master who starves his servant, or as a "normal" master whom Launcelot abandons because of his anti-semitism, which would make Shylock a figure of sympathy for the audience.