What is the function of Abigail Williams in Arthur Miller's The Crucible?

Abigail Williams functions as a main antagonist who drives the play's cataclysmic events forward.  
Initially, Abigail Williams is completely averse to having anybody in the town even whisper about the possibility of witchcraft. She is adamant to Parris that she and the other girls were only dancing in the forest, saying, 

Uncle, we did dance; let you tell them I confessed it—and I'll be whipped if I must be. But they're speakin' of witchcraft. Betty's not witched.

As details about the girls' activities are revealed to the audience, we learn they also drank blood. Many of the girls are very worried, but Abby convinces them to keep quiet about everything. She is determined not to let any rumors of witchcraft survive, threatening,  

And mark this. Let either of you breathe a word, or the edge of a word, about the other things, and I will come to you in the black of some terrible night and I will bring a pointy reckoning that will shudder you. And you know I can do it; I saw Indians smash my dear parents' heads on the pillow next to mine, and I have seen some reddish work done at night, and I can make you wish you had never seen the sun go down!

Had Abigail kept up that kind of attitude, the rest of the play's events would have played out very differently.  
Abigail doesn't keep trying to keep witch rumors away, though. After seeing the attention heaped upon Tituba for naming a supposed witch, Abigail immediately chimes in that she saw other people with the Devil, too.  

Hale: God will bless you for your help.
Abigail rises, staring as though inspired, and cries out.
Abigail: I want to open myself! They turn to her, startled. She is enraptured, as though in a pearly light. I want the light of God, I want the sweet love of Jesus! I danced for the Devil; I saw him; I wrote in his book; I go back to Jesus; I kiss His hand. I saw Sarah Good with the Devil! I saw Goody Osburn with the Devil! I saw Bridget Bishop with the Devil!  

From this point forward, Abigail is the leader of the girls who cry witchcraft, and she accuses whoever she feels like. At first, I believe Abigail enjoys the attention and reverence given to her. Later, Abigail realizes she can use her position of power to have Elizabeth Proctor killed. If Elizabeth is dead, Abigail believes she can resume her affair with John Proctor.