Why does Launcelot want to leave Shylock?

It's not entirely clear why Launcelot wants to leave Shylock, and he is torn internally about whether to go or stay. He states:

"Budge," says the fiend. "Budge not," says my conscience."Conscience," say I, you counsel well." "Fiend," say I, "youcounsel well." To be rul'd by my conscience, I should stay withthe Jew my master, who—God bless the mark!—is a kind of devil;and, to run away from the Jew, I should be ruled by the fiend,who—saving your reverence!—is the devil himself.

It seems that Launcelot wants to leave Shylock because he is a Jew, and in the anti-Semitic world in which Launcelot lives, it bothers him to be "ruled by the fiend" (i.e., a Jew). His head tells him that he should leave Shylock's service but his conscience, interestingly enough, tells him he should stay. Apparently, his conscience understands that he should disregard his employer's religion and remain loyal to him.
However, Launcelot also seems to have issues with his employer. These lead him to help Jessica and Lorenzo elope, actions which are certainly a betrayal of Shylock's trust. In the end, Launcelot does leave Shylock and enters the employ of the fortunate Bassanio who, we are led to believe, is a kinder master.
In summary, it appears that Launcelot wants to leave primarily because Shylock is a Jew, a reason he knows, in his heart of hearts, is wrong. However, he does leave.

Launcelot, Shylock's servant, debates with himself whether to leave his master's service. He's torn between what he calls "the fiend at my elbow" urging him to run off, and his conscience, which with equal insistence tries to persuade Launcelot to stay put. Just to make things even more confusing for the indecisive servant, he reckons he's not just caught between a devil and his better self, but between two devils. If Launcelot leaves Shylock's employ then he'll be following the devil; but then if he chooses to remain then he'll continue to serve someone he openly regards as being a devil. Launcelot doesn't mention any specific grievances he may have against Shylock, so it's a fair assumption that he harbors prejudice towards his master on account of his being a Jew.