"It's obvious because you are my disciple, I don't want my disciple to die that easily. And, there is something I want to tell you, that I am now on the opposite side of you who is a royal soldier, or in other words I am your enemy. I intending to destroy King Vesh... That's why I'm not forcing you to come with me. I also welcome you to return to the Oslesan Kingdom if that's what you want."
Evan actually has no grudge against Eira, but he could not let people who don't trust him stay by his side. If he keeps such a person by his side, it is like a double-edged sword that can both benefit him and harm him.
Evan's goal of revenge would only be shown to King Vesh and the people who participated in torturing him when he was still a Knight in the Oslesan Kingdom.