In addition to the reasons mentioned by my esteemed colleagues above, there is also the vindictive nature and greed of the Putnams which one should consider. Anne Putnam is obviously greatly troubled and aggrieved by the failure of her babies to survive beyond their birth, but she has other resentments as well. One should also consider that since it was so easy for her husband, Thomas, to use his daughter, Ruth, as a tool during the trials, he could not have found a more willing partner to his pernicious scheme than his wife.
Arthur Miller, in his notes, refers to the vindictive nature of the Putnams, specifically as it is directed towards the Nurses:
Another suggestion to explain the systematic campaign against Rebecca, and inferentially against Francis, is the land war he fought with his neighbors, one of whom was a Putnam...
... As we have seen, Thomas Putnamn's man for the Salem min-istry was Bayley. The Nurse clan had been in the faction that prevented Bayley's taking office. In addition, certain families allied to the Nurses by blood or friendship, and whose farms were contiguous with the Nurse farm or close to it, combined to break away from the Salem town authority and set up Topsfield, a new and independent entity whose existence was resented by old Salemites. That the guiding hand behind the outcry was Putnam's is indicated by the fact that, as soon as it began, this Topsfield-Nurse-faction absented themselves from church in protest and disbelief. It was Edward and Jonathan Putnam who signed the first complaint against Rebecca; and Thomas Putnam's little daughter was the one who fell into a fit at the hearing and pointed to Rebecca as her attacker. To top it all, Mrs. Putnam - who is now staring at the bewitched child on the bed - soon accused Rebecca's spirit of tempting her to iniquity, a charge that had more truth in it than Mrs. Putnam could know.
It is clear, therefore, that Anne Putnam's resentment was borne not only out of her jealousy with regard to Rebecca's success in raising babies, but also that the Nurses were seen as enemies. Also, with them out of the picture, Thomas could purchase their land at auction once it has been declared forfeit to the state. The Putnams would have killed two birds with one stone.
Miller's ironic concluding note in the above extract asserts that it is Anne Putnam who turned to evil, contrary to her resolute attempts to prove that it was the other way round.