Why is Candide such an idiot?

I don't believe I would describe Candide as an idiot. Yes, he is naive; he is also easily manipulated and has an overly optimistic view of the world around him. Unfortunately, his circumstances change, and he learns that the world is not beautiful place he'd always believed it to be; he learns about punishment, torture; he learns about anger.

Candide's initial naivety allowed others to take advantage of him. He was trusting, optimistic. and open. Candide had no idea how to respond to the malicious actions of others because he didn't understand maliciousness. After learning fear and understanding threats this changed...... and at times he took his own actions too far.