Before Dorothy came to town, there were two wicked witches. But her house landed on the Wicked Witch of the East, killing her instantly. It's fine, though, really. According to the Good Witch of the North, "She [had] held all the Munchkins in bondage for many years, making them slave for her night and day" (2.16). Clearly, they called her "wicked" for a reason.
The Wicked Witch of the West sounds similarly, er, charming. When Dorothy and her friends first enter her land, the witch literally blows a whistle and says, "Go to those people and tear them to pieces" (12.12). She didn't even know that Dorothy and her friends had been tasked with killing her! She just hated their faces that much.
Throughout the rest of what would prove to be her short life, the wicked witch is similarly nasty. For one thing, she tries to kill Dorothy four more times. And when that doesn't work, she finds a way to enslave the girl. All the while, she harbors intense jealousy of Dorothy, whose silver shoes have special powers. She plots to steal them, going so far as to trip Dorothy to knock one of the shoes off.
But beneath this thoroughly unpleasant exterior, the witch is a weak, scared woman. When she realizes how powerful Dorothy is, "she trembled with fear" (12.59). In fact, she seems afraid of a lot of things, including ordinary stuff like the dark and water. ("The Witch was much too afraid of the dark to dare go into Dorothy's room at night to take the shoes, and her dread of water was greater than her fear of the dark," the narrator tells us (12.72). Soon we learn that this last fear, at least, is based in reality. The reason that the witch fears water is that she's deathly allergic to it.
In death, we see just how far removed the wicked witch is from humanity; water, the very thing that sustains human life, is fatal to her. Other traits, too, make her seem like something less than human. When Toto bites her, "the Witch did not bleed where she was bitten, for she was so wicked that the blood in her had dried up many years before" (12.70). But by far her most inhuman trait is her cruelty. The wicked witch is awful to everyone—even the Winkies. How could anyone have the heart to enslave a species with such a silly name?