>>714207165
It got corrupted by Soul Edge to some extent when Xianghua left it stabbed in Inferno between I and II. That's why it's got that evil variant in II. It did get purified later, but some people think it didn't fully work.

The more popular theory is that it was always evil. Soul Calibur was made from a shard of Soul Edge by Algol, for the purpose of opposing Soul Edge. That ought to mean that other than hating Soul Edge, it has the same nature as Soul Edge.

Soul Calibur is a lot weaker than Soul Edge, so Soul Edge is usually the one threatening the world. Soul Calibur always appeared to be good because it was opposing the sword that's threatening the world, but that's not because it's the good sword. It's just the less threatening evil sword, which has the common goal of destroying the more threatening evil sword.