In ancient India they were very very refined, they used to choose girls and poison them from their childhood, they were called vishkanyas (poison girls). From the very first day of their life they started given them poison in very small doses and only very very slowly those doses were increased.
By the time they became young women their whole blood was poisonous. Even snake bite will not kill the girl. Then they chose the most beautiful of those girls because actually they were raised to be agents against the enemy. When one of those agents managed to the neighbouring kingdom, her overwhelming beauty opened the doors to the king.
One kiss of the girl and that was enough. The king was finished!