I mean would you want a dangerous shitty man chasing your brother? He didn't have a problem with ilay before because he himself knew it was just the roles they played and that he really couldn't harm him. I mean so many people have come after him all his life so he doesnt bat an eye to it anymore. But when it's his brother he's obviously going to care even though from an outside perspective it may not seem like it. His brother cares a lot about him which is why here he stopped making weapons because he knows taeui hates it and doesn't want to disappoint him further.
Just to clear both of the other comments . He doesn’t hate Ilay because “he is chasing Tae” . He hates Ilay because in this conversation he can guess that Tae has feelings for Ilay . And he feels like Ilay will “take his brother” from him . He will not be Taes priority anymore . Later in the novel the same happens with Tae and Ranhma. Tae really dislikes Ranhma , and he hates him for “taking away” his brother from him too .
Yes yes to this, I would love to say that he's a normal brother who hates ppl who endangered his brother tae but no, jaeui has been the one who actively goes out of his was to avoid tae bc he wants tae to chase after him regardless of what that would mean for tae's safety.
I think I remember that in the novel with jae and Rahman, jae actively thinks like this too.

Idk why he said he is starting to hate that Ilay is chasing Taeui. He said there's no reason to hate Ilay when they first meet, but when he learned the chasing, he said "I might start to hate it." Well this is a rough translation for chap 127, and I just google translate it, but still.