This serpent is the offspring of Echidna and Typhon so it was born to hate humans.
The Lernaean Hydra was a vicious gigantic water serpent with nine heads, it's largest head being immortal whilst its eight smaller heads could be injured during battle, however when one is severed, two more grow in it's place which makes it stronger against its opponent.
All nine heads have razor sharp fangs that are always filled and overflowed with deadly venom, this makes their breath extremely poisonous and could kill anything on it's territory.
Hercules killed this beast with his companion Iolaus. When Hercules severed one head, Iolaus would sear the flesh to prevent more growing. Hercules then buried its immortal head deep underground.