The answer to your question depends largely on who you have determined was to blame for the tragic events that happened in Arthur Miller's The Crucible. There is plenty of blame to go around, so here are several ideas since I'm not certain which might suit your own ideas.
First of all, the Puritan system could be blamed for this tragedy. It is a religious culture that no doubt began with spiritual motives; however, by the time we encounter it here, the rigidity of sin-finding and sin-punishing have created an environment of paranoia and suspicion. People are much too quick to find fault in others, hoping this will divert some attention from their own flaws. And don't forget, this all started because, according to Puritan law, the girls were going to get whipped for being in the forest one night. If this is your position, your topic sentence might read something like this: The primary cause of these tragic events in Salem was the Puritan theocracy which demanded legal and religious punishment for sin, rewarding those who claimed to recognize evil.
Another source of blame for what happened in Salem is the court. It is true that the judges had a job to do and they took it quite seriously; however, they also consistently assumed the accusers to be true and the accused to be guilty. In fact, we know that the judges even ate with the accusing girls, as Mary Warren says.
Four judges and the King’s deputy sat to dinner with us but an hour ago.
Even in the face of evidence and testimony, the judges proceed; and it is evident in the last act that Danforth is too proud to relent in any way, even though he has compelling reason to do so.
I will not receive a single plea for pardon or postponement. Them that will not confess will hang. Postponement now speaks a floundering on my part; reprieve or pardon must cast guilt on the guilt of them that died till now. While I speak God's law, I will not crack its voice with whimpering.
If you believe the court and judges are primarily to blame, perhaps a sentence like this would be effective: If the judges of the court had not been too prideful to consider that they might be wrong, and if they had been more open-minded about the testimony and evidence brought before them, lives could have been saved.
If, however, you blame Abigail as the instigator of the trials, which is a reasonable position, your topic sentence might be this: Abigail Williams was the cause of the witch trials because she wanted to avoid punishment for dabbling in witchcraft and because she wanted John Proctor for herself.
Abigail's selfishness and deceit are the root of everything else, though of course others are not blameless. Parris certainly promotes the trials because it benefits him not to have anyone questioning the behavior of his daughter and niece. The Putnams obviously benefit financially from the trials, and the other girls are just glad to have avoided a public whipping.
Perhaps you have something else in mind, but this should give you something to work with as you write your own topic sentence.