Firstly, the author shows us that the Duke is crazy because he said “Then all smiles stopped together.” The author could have been inferring to us that base on the words of the Duke that he could have possibly kill or send the Duchess to a mental facility because in this era it was common for this like that to happen and by doing that he won’t have to see her smile towards anyone else ever again. This is how the author revealed to us that the Duke is crazy in My Last Duchess.
Secondly, the author reveals to us the the Duke is a Jealous person. “This grew; I gave commands.” This line shows how the Duke is jealous towards the Duchess because