The Duke only said that Ilya von Bismarck should be brought to his office in a couple of week's time. Hence, Vhelian wanted to circumnavigate the said mission.
He detested the very existence of Ilya. The sole way he could accept the mission was if he would be allowed to kill her in every way.
Despite not having the confidence to confront Ilya one on one, considering that he had heard how powerful of a Sorcerer she was, the young man would request the help of the Dukedom's soldiers in order to subdue her. After all, they were in possession of chains that could hamper a Sorcerer.
Be that as it may, he made the wrong decision of informing the Duke about what he had in mind. It would've been better had he just went along with his whims and created a situation where the Duke wouldn't be able to do anything but regret.