Sarah Good was the town drunk. She was an older woman and was often seen going from door to door begging for things. She was probably homeless and living off the kindness of others.
Abigail accuses of seeing Sarah Good with the devil, among others. Sarah was an easy target for Abigail. Sarah lived on the margins of society, so she was the perfect person for Abigail to accuse. She was a mentally unstable woman, so the town didn't care what happened to her. Sarah eventually confesses to witchcraft to save herself from being executed. This was the only way for anyone to save themselves. They had to admit to something they didn't do. People knew that Sarah was not a witch, but they knew she was trying to save her life.
What is so very sad about Sarah is, that most of her life had been spent living in despair. She was poor, had mental issues, and was a drunk. Abigail had no problem at all accusing this poor woman of something so untrue. Abigail just continues to abuse Sarah, as everyone else has in her life. Sarah Good, makes us stop and think about how we treat each other. The only Sarah had left was to admit to something she didn't do, just to save herself and continue living the life she had always lived. That is the real tragedy.