Rabbinic literature in general, both early and late, has little good to say about tax collectors, and considers them to be blatant sinners. The tax collectors spoken of in the Gospels served a foreign government that did not have the manpower to execute the enormous task of gathering taxes in all the provinces of their far-flung empire. Their fellow Jews in the province of Judea saw tax collectors as collaborators who enabled the Romans to continue to impose their conquest over the land of Israel.
Furthermore, even if the Romans had been able to send their own tax clerks, these would not have been as effective as locals in collecting the taxes. Local Jewish tax collectors could not be fooled as easily as foreigners since they knew the vernacular and what was taking place on the local scene. Thus tax collectors were resented all the more. One’s neighbor just shouldn’t do such a service for the conquering power.
Tax collectors were especially hated because they increased their profit by collecting more taxes than their masters actually demanded.