How can a judge in a lawsuit against a governmental entity be totally fair and impartial, when the judge is employed by a governmental entity?

If someone was suing the federal government, for example, the case would be heard by a federal judge, who is employed by the federal government. And this same judge, as I understand, has taken a solemn oath to, “preserve, protect and defend the United States of America against all enemies, foreign and domestic” (or something similar to those words). So wouldn’t it be understandable to question where his/her loyalties might lie? I just don’t think that anyone can be all that “even,” if they are employed by one of the parties involved in the litigation.

My suggestion would be that in such cases, the judge’s salary for such a case would be paid for by both litigants, and the judge would no longer have a suspected loyalty to one side, over the other.