From my understanding, it seems the short pant killed his cat since that cat is close with the commoner and infected with the disease, so he got no choice to kill it (and since he is healthy enough, maybe he's not infected)
Then he somehow push the slave to take her shoes in the castle (via telling her she better need shoes), thus when the slave visit the castle with the commoner dead body, she might be responsible in spreading the plague in the castle so its only matter of time everyone in the castle and city got infected with the black death
All of this can be avoided if the princess not so fixated with war and too arrogant, also make the commoner and slave as fugitive, so this is also karma for the princess
well, thats what i got