Sonja had seen how the creature responded to anyone approaching intending violence. The skulls and bloody water in front of her were where the two hired thugs had been killed messily after they had charged in waving their swords.
She had been hired to help the town, as the tentacle critter in the lake had been molesting the women (and several said it had molested them day after day, with a faint smile), while it had been killing the men who had tried attacking it. Sonja had seen the obvious way to deal with it, but two nearby thugs had only thought of being the heroes to a town with far more women than men. Seeing a way to get rid of their stupidity, Sonja had agreed to a three way split of whatever reward the town had.
The thugs had been only thinking of their 'reward' as they had charged, and had gotten the results. Now Sonja was going to deal with the creature, and after it was weakened then she would kill it. After that, she would return to the town for the reward. Her oath said she could not give herself to any man unless he defeated her in battle.
It said nothing about women, and there were a lot of lonely women in the town.