"...if from a positive bias towards one person/people/country necessarily follows a negative bias towards others."

A bias means a built-in tendencyt to go one particular way. That means by definition a built-in tendency not to go the other way.

In any conflict situation a bias in favour of one side has to mean a bias against the other side. Perhaps you are taking bias as meaning something equivalent to "liking", or having sympathy for - and it would would indeed be quite consistent to like both sides, or to have sympathy for both sides, and to ber ipartial. But bias cannot be impartial by definition.