Don't know really. SC masked itself as the Krita-Yuga for a very long time so whether there's a true holy sword out there isn't really known. I think SC was known as the holy sword since it was purified by the Zassy's tribe and the Dvapara Yuga as well as the Kali Yuga kept a hold on its powers, thereby forming the trio of holy item.
Nice observation Pyrrus. That's very similar to my line of thinking. For me, here's how Soul Edge and Soul Calibur manipulate its wielders.
Soul Edge----> Fears. The sword preys on the wielder's fears to manipulate them. Siegfried killed his father and from the immense guilt, he lied to himself and believed that his father's killer was out there. After obtaining Soul Edge, the sword fed into his fear by making the deal that if Sieg gave it more power, it would resurrect his father. If his father came back, his sin would be gone and everything will be well. He won't need to fear the haunting truth of the horrible murder he committed. Pyrrha was afraid of being alone and Soul Edge promised her it would stay with her forever if she gave it souls. But yes, in the end, it destroys the wielder's mind and makes them insane like you said ;D
Soul Calibur---->Lies of salvation. This is tricky since it's not very touched on. Pat wants to save his sister and SC seems to have twisted the words by saying that by giving his sister 'death', will she be saved and found peace. He himself is convinced he's saving her.

:Those who wield SE are enemy of mankind. I will save you and protect this world.
His mind isn't clouded really. SC just made him believe that he is saving Pyrrha and the world by killing her. For comparison, it's like Le Bello using his silver tongue to trick someone xD
Conclusion, she offers her wielders their 'wish' in a very twisted way and that's how she controls them.
Soul Calibur seems to be infatuated with the idea that death is salvation and continually calls herself the light of humanity or something. Where SE represents chaos, she represents order. True order is death, ironically. There's no lies, no anger, hate, wars, etc if everything in this world is dead. A world that's icy cold and devoid of life is her twisted idea of peace and salvation. The two sword's methods in the end is the same really, just lies.