Yuri was terrorizing teacher while justifying that he was doing this for noona. But as we know, he's actually doing it for himself. He's clinging to this idea that noona isn't in an honest relationship, that she's being lied to, and in a way wants to save her from the relationship and get her "back." So now with this conversation and lack of reaction from his brother in law, theres a sort of realization that nothing really was going on and he's left still jealous and upset, so he lashes out.
Yuri was terrorizing teacher while justifying that he was doing this for noona. But as we know, he's actually doing it for himself. He's clinging to this idea that noona isn't in an honest relationship, that she's being lied to, and in a way wants to save her from the relationship and get her "back." So now with this conversation and lack of reaction from his brother in law, theres a sort of realization that nothing really was going on and he's left still jealous and upset, so he lashes out.