The fact that John Proctor is married makes his affair with Abigail entirely wrong in the Puritan society. In addition, Abigail likes to make many claims about Elizabeth. It is what spurs her desire to get excited about drinking charms and the like. She wants to do something to hurt Elizabeth. In Act 1, with the chance to talk to Proctor about Elizabeth she says:
"She is blackening my name in the village! She is telling lies about me! She is a cold, sniveling woman, and you bend to her! Let her turn you like a - "
This shows that Elizabeth functions as the jealousy of Abigail which in turn encourages her to work toward separating John and Elizabeth. John finally confesses this to the court by the end:
She thinks to dance with me on my wife's grave! And well she might for I thought of her softly.
When John confesses this in Act III, he admits that his marriage to Elizabeth was insecure and that Abigail had a motive to kill Elizabeth. John was trying to get away from it, but Abigail kept after him. In this society, Abigail had little other chance to do anything because divorce was not an option.