Um, well, the serpent was responsible for tempting Eve to eat from the Tree of Knowledge of Good and Evil, which God had forbidden. The serpent told her that it would bring wisdom, or let her be more like God, but it resulted in... the loss of innocence. Adam and Eve were banished from the garden, cursed with a life of hard labor and pain during childbirth, respectively.

The snake was made to crawl on its belly and, and forever be an enemy of mankind. The details and interpretations vary, of course.