There is no single answer to this question, but there are a few contributing factors that we can observe.

The first is that a Puritanical philosophy often involves interpreting the Bible quite literally. Those who adhered to the religious standard of faith in Salem no doubt believed that the Devil was a tangible, real entity along with every other entity and concept in the Bible, and witchcraft is certainly mentioned.

The teachings of certain religious leaders are also thought to have "set the mood" for the Salem Witch Trials. A Puritanical minister by the name of Cotton Mather is credited with publishing Memorable Providences, a book that detailed the afflictions and fits of certain Boston children. He argued against scientific explanations for the mental and physical illnesses of these children and blamed witchcraft and the Devil instead. Memorable Providences was published three years before the witch trials and is often credited by critics as being instrumental in creating the witch-hysteria that allowed for the trials.

Lastly, as with most tragedies that have occurred throughout history, the core of this great collective evil was fear. Even the most religious among us tend to think of the Devil in more abstract terms in our modern age, but to the Salemites, this entity represented a horrible and tangible fear that infected their daily lives as rumors of witchcraft began to surface. When faced with someone accessed of witchcraft, fear of that magnitude caused the people of Salem to lash out with mob mentality, and there was little room for reason or logic to be heard. In fact, this fear was so effective in uniting the people that some conspiracy theorists even speculate that the witch trials themselves were a ploy by the clergy to cause the population to become more spiritually dependent on the church.