I think that with any human being (or character based on a human being) that no one is all good or all evil. The same holds true in The Crucible.
It's easy to say that Abigail is evil, as she is the one who set the whole frenzied witch hunt in motion after being spurned by her lover, John Proctor. However, remember that she was a young girl who entered into an affair with a married man who threw her over when he decided to go back to his wife...so in a way she was betrayed as well.
John Proctor was a good man...choosing in the end to do the right thing, but he did break his marriage vows...so he too could be seen as evil by some.
Tituba could be seen as evil, as she conjured up magic spells and encouraged the young girls of the town to prance naked around the cauldron in the forrest.
Mary Warren lied and accused Proctor of making her sign the Devil's book when she can't handle the other girls' accusations any more. That was an evil deed, but did that make her an evil person?
How about Judge Hathorne (FYI- a distant relative of Nathaniel Hawthorne) who sentenced women to their death on the scant evidence of the accusation of a few wild girls.
But I think that's one of the points of the play...that the lines between good and evil are blurred...that life is not black and white...but subtle shades of gray.