Even though he was overall meant to be a sympathetic character and in a distressing situation...
... it was rather immature, unprofessional, hurtful even and rather wrong of Marcus' character to confront Klara in the ways that he did, and he also didn't know everything behind the story to confront the kid like that and call her names etc.
Do we agree?
Even Lukas himself when he found out what happened didn't get mad at Klara like that personally.
Though I kind of suspect, the film doesn't want to say that what he did in that situation was exactly "right" but he was dealing with a very distressing and emotionally as well as physically confronting situation for him and in that sense he became for a while briefly blinded by personal rage and Klara in a sense was like an easy target for him, though it was both wrong, counter-productive and incredibly immature of him like that. And he didn't even want to listen to Klara (she even says "I didn't say anything" and he continues "What are you saying about my dad?" and its like he assumed that the child intentionally (and not via any misunderstandings etc) said something about Lukas like that with "full intent" to bring all those troubles on him, or maybe he was just upset by the suddenly occurring aftermath like that.
He should've really confronted probably Grethe like that or that man who botched the interview.
But still. And with that spiteful s..., you know. I ALSO WONDER...
Regardless of how the situation may have ultimately turned out, it may be possible that at least in a few years, most people in that town ultimately probably DID discover what really happened and what caused Klara to utter that confabulation and why it was turned into a threatening truth that led to vigilante witch-hunt. But I wonder, what did Marcus think of it, and let's say he found out for instance that Klara's brother's friend shown her inappropriately indecent material via the ipad and Klara had a brief quarrel with Lucas, would Marcus also confront him and maybe even state "Look at what you've triggered!" etc. And let's say Marcus met Klara again, maybe even apologized but could he also be like "Yeah, with these things, we have to be careful and not say that etc" even as children. But anyways...
Marcus and him being mad at Klara - offensively wrong and terrible and unforgivable etc or undeniably wrong, immature etc but understandable via the circumstances, cheers.