Why is it OK for atheists to call Christians "Bible thumpers," but wrong for me to call Jews "greedy Shylocks"?

Greedy Shylock is a double insult. Bible thumper is neither. Shylock is a derogatory name for a Jew, and greedy is adding insult to injury. Bible thumper only implies a believer's impetuousness or arrogance. Neither, IMO, are nearly as offensive.