This is a fairly interesting question. I don't think that Miller depicts her as a saint or anything like that, but he is able to evoke the complexity within her character. This does not excuse her from what she has done, but Miller's composition makes her a pitiable figure who does awful things. The fact that she was denied a parent's love and lived with the truth of their brutal killing cannot be underestimated in terms of its impact on her ability to give and feel love. Parris does not really serve Abigail's emotional needs and, in the end, she does not fully understand what it means to love and care in an open and transparent manner. With such a background, she embodies the moral corruption and ambiguity that is so rampant in Salem. Presumably having left to become a prostitute, Abigail is someone that evokes some level of pity. She is a sad figure, who does some awful and repugnant things.
No, I don't believe Abigail is a sympathetic character and, in fact, I think the audience grows to despise her actions and manipulations of others throughout the play. When she steals away under the cover of darkness near the end of the play, I think most readers probably silently celebrate that she is gone and some probably wish for some evil to brought onto her for the evil she has brought onto the people of Salem.
Throughout the play, Abigail thinks only of herself. Her manipulations of others are all contrived to help her gain that which she selfishly seeks. Whether it is the seduction of John Proctor, her manipulation of the other girls and the court, her betrayal of Mary Warren, etc, we see a long line of immoral and criminal acts against her community. Many, many times Abigail had a chance to acknowledge her mistakes and begin to reverse the damage she had. She refused. And because of this, I think there is no excusing her actions.
I don't believe Abigail is sympathetic. If she was not quite so evil and manipulative, then possibly. However, she lies about person after person day after day. She could have "repented" her own sins without pointing fingers at others and causing innocent lives to be taken. She was particularly heinous in her false accusations toward Elizabeth and Mary Warren. She is motivated by revenge, self-preservation, and spite. The other girls were really her followers, so she abused her powers of persuasion.
Modern readers often point out that she's practically a child, and that John Proctor should not have taken advantage of her. I have heard students argue that she "did it all for love." However, she runs away like a thief in the night at the end, and that is not very romantic or redeeming.