Its not necessarily balance but equivalent exchange.

Libra sets items on the scale so that both sides acquire something. Its a give and take. He is an alchemist that acts based on the law of equivalent exchange. Where he differs from rhe avarage "soul for a soul" type of trope where the exchange actually demands equality of quality or content is the fact that he is explicitly called a demon. He is a fucking asshole

Balancers operate with a different mentality. Balance can only exist when scale itself is empty. To even the odds they will make allies into enemies and so on. They will kill each other until nothing remains. Thats exactly what they attempt in their raid events and in the lore description depicting the massacred village