This one's pretty obvious. The evil, tyrannical Witch who likes to turn people into stone and murders an innocent victim is the baddie in the story. Was anybody not clear on that one? OK, we've cleared it up.
The more surprising bad guy in the book is Edmund. Edmund starts out as an annoying jerk, develops into a traitor, and then has a complete change of heart and becomes one of the good guys. While he's functioning as an antagonist, though, he's even more dangerous than the White Witch.