Just an FYI, it's a he. Cadmus is the fabled founder of Thebes.
He slew a great water-dragon, after it killed his comrades, who came to the dragon's wellspring, to gather water for a sacrifice to Athena.
Athena instructed Cadmus to "Sow the Dragon's Teeth" in the soil around the spring. In time, the teeth "sprouted" into a warrior race of men known as the "Spartes". Cadmus enlisted them in helping him build the Cadaium - the temple at the heart of Thebes.
Afterward, Cadmus was sentenced to eight years of punishment by Ares, for the crime of killing his dragon. When Cadmus was finally released, he was wed to Harmonia.
There is TONS of great symbolism and allegory in this story. Athena is the goddess of Wisdom, Peace, and Reason -- but she is also the Goddess of War. Ares is the God of Bloodlust (War Throng). So, the painting could be symbolizing a struggle between reason and passion?
Water is often used in mythology as a symbol of cleansing, or change (ergo, the phrase "Watershed moment").The slaying of the water-dragon is supposed to be one such symbolic moment of great change (as far as I can remember). So, perhaps the artist was trying to say something symbolic about the time when this work was done?
I also wonder what the painter's intent was, with the cloth that Cadmus is entwined in. It looks very serpentine. Also, Cadmus' legs seem "bound" in it.