The Peddler got the idea of the world being a Rattrap from his circumstances and miserable life condition. This can be inferred from the events and incidents that take place in the story.

Introduction:
The story "The Rattrap" by Selma Lagerlöf revolves around a Peddler who is a homeless wanderer. He lives a miserable life and is constantly struggling to make ends meet. The Peddler's experiences and interactions with other characters in the story lead him to perceive the world as a Rattrap.

Interaction with his friends:
One of the significant incidents that influences the Peddler's perception is his encounter with two of his friends. His friends, who were once thieves, had also led a similar life of poverty and despair. However, they eventually found happiness and contentment by leaving their criminal activities and settling down as farmers. The Peddler sees this transformation in their lives and realizes that there might be a way to escape the vicious cycle of poverty and despair.

Observing Crofter's behavior:
Another incident that contributes to the Peddler's perception is his interaction with the crofter, who offers him shelter for the night. The crofter's behavior towards the Peddler is initially welcoming and kind. However, the Peddler soon realizes that the crofter is suspicious of him and treats him with mistrust. This behavior makes the Peddler question the true intentions of people and reinforces his belief that the world is like a Rattrap, ready to trap and exploit unsuspecting individuals.

Edla's words:
The final incident that solidifies the Peddler's perception is his encounter with Edla, the daughter of the ironmaster. Edla treats the Peddler with kindness and empathy, offering him food, warmth, and companionship. She engages him in a conversation and expresses her belief that deep down, everyone is good and has the potential to change. This conversation has a profound impact on the Peddler, as it challenges his cynical worldview and offers him a glimmer of hope.

Conclusion:
In conclusion, the Peddler gets the idea of the world being a Rattrap from his circumstances and miserable life condition. His interactions with his friends, the crofter, and Edla, along with his own experiences of poverty and mistrust, shape his perception of the world as a trap. Through these experiences, the Peddler learns that kindness and empathy can break the cycle of despair and transform one's life.