Abigail and John Proctor had an affair when she worked for the Proctors, and she believes that John loves her. In her mind if she can get rid of Elizabeth Proctor, she can become John's wife. This angle is one of her motivations.
Her other motivation is her need to control. When the girls are caught in the woods and Betty Parris fakes a catatonic state, Abigail has to come up with some explanation for what has caused it. The men accuse Tituba of witchcraft when it becomes clear that the girls were engaged in a ritual led by her. Tituba, of course, pleads that she did only what the girls asked, but when she is threatened with death, she confesses and even names others involved with the Devil, once other names are suggested. Abigail sees how much attention Tituba is receiving and perversely wants that concentration directed to herself; she has a need to control others, and she is successful. As the play unfolds, she will control the girls' behavior as well as that of most of the adults'.
Abigail is clearly the villain of the entire play. Her accusations sent 19 innocent people to their deaths. Her entire reasoning behind this, is selfishness.
Abigail was raised as an orphan after her parents were killed by Indians. She went to work as the housekeeper for John and Elizabeth Proctor. Abigail and John Proctor end up having an affair and she is convinced she is in love with him. Elizabeth finds out about it, and dismisses her from the job. This makes Abigail angry and very jealous of Elizabeth. Her desire to be with John, leads her to do some horrible things.
"I look for John Proctor that took me from my sleep and put knowledge in my heart! I never knew what pretense Salem was, I never knew the lying lessons I was taught by all these Christian women and their covenanted men! And now you bid me tear the light out of my eyes? I will not, I can not! You loved me, John Proctor, and whatever sin it is, you love me yet!"
Clearly, Abigail is quite delusional. Yes she and John had an affair, but it is never said that John told her he loved her. We just don't know this. Abigail is bent on getting Elizabeth charged for witchcraft and executed, so she can take her place and become John Proctor's wife.