The Merchant of Venice is full of contradictory characters and things are never quite as they seem. Shylock, Jessica's father, is a hard man with little room in his heart for compassion. He makes no secret of his dislike for Antonio - in fact all Christians - "If a Jew wrong a Christian, what is his humility? revenge"(III.i.60) and rationalizes that, as the Christians are cruel to the Jews, the Jews will measure it right back at them: "The villany you teach me I will execute; and it shall go hard .." Shylock is quite clear that "I will better the instruction." (63). Therefore he does not endear himself to others and they feel disdain towards him, no less because he is "a Jew" and " will feed fat the ancient grudge I bear." (I.iii.42)The animosity between Shylock and Antonio is almost tangible (so real you can touch it).
The audience would have related to Jessica's desire to free herself from her father and her religion as they disliked Jews who had in fact been banned from England for centuries before. The audience would have applauded her wish to convert to Christianity even more!
It is only when Shylock talks of his treatment at the hands of the Christians and mentions Leah, from his "bachelor" days and the ring she gave him, that we are inclined to feel pity for him. Up until now, he has shown little need for human companionship and his daughter has seen nothing of his love for her so her betrayal is not a surprize. Shylock is viewed as a far more complex person by the end of Act III scene i.
It seems there is more to Jessica's betrayal than meets the eye and it is perhaps her need to hurt Shylock more than a real love of Lorenzo and a real committment to Christianity. She steals and sells Shylock's beloved ring, probably more to render a reaction from him and hurt him than anything. Lorenzo is her escape and "hath made me a Christian." (III.v.16)
There are others who feel that she can never escape her birthright and the fact that she is a Jew will always count against her as "the sins of the father are to be laid upon the children"(III.v.1) and Jessica is "damn'd."(7)
Jessica's betrayal is therefore understandable in the light of such conflicting opinion and little compassion for her although she may not be received by the Christians as readily as she would like.