I think she did that AFTER being subjected to the Slave Crown, to test her powers. Or, as you said, she could have lost control of her powers and killed those 50 soldiers, hence the Slave Crown. I hadn't thought of it that way.

From the same game, Celes was a general and high-ranking in the Imperial army, but she then changes her mind and defects, but it is never fully explained WHY she changed. It is only said that she "realized the war was stupid." She was fighting in this war, so what happened that made her see the stupidity of it? Why is this never explained?