Another major turning point in the play that seems to have been missed in the initial answer is not only Tituba naming names, but even before this when Abigail turns Tituba into the first scapegoat. It is at this moment that we see the weak will be preyed upon and it will also be accepted my the majority who are looking for someone to he responsible for the indiscretions of others.
Miller we makes this point to show how the HUAC did the very same thing when they needed to blame people for something. They needed to look proactive and or sent in the problem rather than looking as if it was being ignored. Miller's play mocks the moves of McCarthy. Hence why Abigail, who is a representation of the HUAC, blames the slave who she knew couldn't save herself. In addition, Miller points this out in his description of the actions of Abigail regarding Tituba in Act I.
There are several instances where turning points can be seen. One of the most signifiant turning points would be in the ending of the First Act. It is at this point in the drama where Tituba panics and "names names." Once the girls see Tituba receiving public praise for naming names, they begin to do so. This moment is a significant turning point because the practice of "naming names" and thus adding to the hysteria of Salem becomes evident.
Another turning point would be in the ending of Act III. The courtroom scene where the girls exert their power over Mary Warren is significant for a couple of reasons. It shows the power that Abigail and her crew hold over everyone in Salem. They are able to manipulate the courts, the civil and religious leadership, and essentially work the system to their own benefit. This was seen in how they get Mary Warren to capitulate and recant her accusation against them. Another reason it is a turning point because John Proctor realizes that he cannot win. He understands that the force of goodness present in people like himself, Goody Nurse, and Giles Corey will not be able to overcome the sinister forces of the girls. It is a turning point because Proctor admits to his affair and confesses that he is not able to be silent and remain isolationist any longer. This is a turning point because Proctor recognizes the strength of the forces against him and the forces that are running Salem into madness.
The final set of turning point can be seen in the drama's end. When Proctor initially signs the confession and then recants because of his "name," it is the critical turning point in the thematic development of the drama. Proctor's goodness, as Elizabeth puts it, and his sense of honor in a world filled with dishonor is a significant turning point in the drama. It is a turning point because it moves Proctor from an ordinary human being into a supremely transcendent figure. This is a turning point because it marks the point in which Proctor, himself, changes. In doing so, he provides an inspiration to those around him and the audience, as well.