This question is about "The Crucible"
Answer:
Because his family was a victim of witchcraft.
Step-by-step explanation:
Thomas Putnam believes that his family was a victim of witchcraft and this was done to denigrate the Putnam's name in the society in which they lived.
He believed this, primarily because his brother-in-law had been barred from being the minister of salem, all of his children (with the exception of Ruth) died after birth and Ruth was found in the forest dancing with abigail and other girls something that seemed pagan.
In the face of these facts, he believes he has been humiliated and decides to participate in the salem trials and to accuse those he thinks were responsible for diminishing him.