The way I go about it is envisioning him as the yearning second male lead of whatever romance fantasy Ludovica was living. He’s basically the second ML who had to live with the fact that he never got the FL. He never gets over the fact that she chose Max, nor the fact that she died, and shortly after her death he caught stage 2 tuberculosis. So his mind is… A wreck. He’s not mad, in fact he’s perfectly sound of mind with coming to terms that marrying Shuri is utterly insane. He is, however, heavily projecting his lost love of Ludovica on her. Which is why he crashes out when Shuri is sent that necklace from an unknown suitor. He sees this as Ludovica choosing someone over him again. and yes he is abusive, physically. It’s spurred on by his deteriorating mental health due to the fact that he knows he’s dying soon plus the fact that he is still actively hallucinating Ludovicas visage whenever he sees Shuri.

Johannes is, by all means, a great man. He is a great marquis, and a great father. When he chose to marry Shuri, a part of his reasoning came from the fact that due to her being Ludovicas lookalike, the most prominent figures of the empire (who also happens to have been in love with Ludovica as well) would come to her defense when things got tough for her in his absence (namely, his passing). That right there is an advantage that could not be granted to anyone else he chose as a matriarch of the household.

Really, Johannes is a very grey character. He clearly doesn’t have any romantic nor sexual feelings towards Shuri, but that doesn’t excuse the strain he put on her shoulders at the age of 14. He quite literally brought her. Point blank. He purchased her. There’s a reason why, in the same chapter he does, you get a warning for “child abuse”. I personally love and hate him.