Dahut, daughter of King Gradlon, who built the glorious city of Ys for her.
Ys, where she could safely continue her pre-christian life-style.
Laughing in the face of the Christian Guénolé and worshipping her Old Gods, whom the Christians call "devils".
Now can you imagine a single reason for Dahut to flood her beloved city?
Think about it.
As I see it, there was but one person involved with enough hate to open those damned gates and drown a whole city's population of "unbelievers".
And if that was not enough, that same bigot convinced Gradlon in his despair, to throw Dahut from his horse into the raging sea and thus condemning his own daughter to her death.
Afterwards, Guénolé was proclaimed saint.
Sometimes, even stories can make you sick.