Dietfried is acting the way he is towards Violet because when he first found her and brought her along, she killed a bunch of Dietfried’s team which I believe is mentioned in a previous episode. That’s why he kept saying how he is disgusted by the fact that Violet is writing letters that bring people together using the hands that killed so many people, including his men.
And as Adam mentioned, I think he is blaming Violet as to why she’s the one who survived from Battle of Intense instead of Gilbert.
Also this is just my theory, but I think Dietfried is afraid of Violet to a certain degree and he’s disguising his fear with anger and loathe.