According to the words of the widower Duke, the Duchess, as lovingly and smiling as he would portray her was also quite salacious and flirty with people form court. When we get to the point where the Duke explains her demise, we encounter an enraged and hurt man who may have been cuckolded, and humiliated. In the Duke's own words, he made the order to kill the Duchess.
Hence, we read the words
I gave commands. Then all smiles stopped altogether.