hmmm.
From the clues I am assuming that the merchant purposely goaded the princess into spending overly lavish amounts on the party to start crippling her reputation. Now that she can't marry the duke she is fairly useless to him and almost a threat at this point. If he's able to move his focus on controlling our MC he'd have power over the duke and the emperor and strengthen his power. He doesn't particularly want someone smart like the princess to be in power, he wants someone he can control.
she's a true anime protagonist. "You think killing me can kill me? Think again"
Decapitation is just a scratch in this girls book. Truly protagonist move