Where did you come to the conclusion that it was to harm Ferrius? Myard was supported by Ferrius, the manga even clearly states that it was the combined forced of Myard and Ferrius that trampled the Canoa country (chapter 1, page 32).I never denied what was mentioned about the king. The ravaging of the lands is clearly mentioned in the manga, although there is nothing that implies any of it was done to harm Highserk. If anything, it's actually implied it was done to harm Ferrius instead, since that's with whom they were at war at the time. The negative emotions of the Myard people also are not something I ever put into question. You're arguing points I never touched
And its only logical why they did it. Canoa lost the war, the duke betrayed his nation and retreated towards Ferrius establishing his own nation. Highserk now had to deal with the scorched earth in the territory that they annexed.
All of these things are stated in the manga.