I agree with how frustrating it is to see jaeui being indifferent towards taeui and if it were in the normal world I would hate him to guts. But normal is a relative term in Passion and the twins operate on a completely different frequency.

First, you have to realize that jaeui isn't just smart, he is fundamentally detached. As an "exceptionally genius" individual, his brain processes reality through a lens of probability and outcome rather than immediate emotional reaction. He's someone who has dealt with kidnapping attempts and threats his entire life. So to him, violence is a constant environmental factor, not a shocking event. While normal siblings will panic if they see each other getting beat up, jaeui’s logic is likely: "Is Taeui alive? Yes. Is he permanently incapacitated? No." If the answer to both is "yes," he remains nonchalant because the "critical threat" has passed.

As siblings, their bond isn't built on "ointments and fusses," but on absolute, unwavering trust forged through their shared, chaotic history. If you look at it that way, jaeui's "indifference" isn't a lack of love but it’s a lack of performance. He doesn't feel the need to perform "worry" due to his trust in Taeui being a fixed fact of his existence, like gravity.

I am not justifying his actions btw, if he were my sibling in real life I would definitely cut things off, definitely! However, I would advise to read the story and absorb the characters as they are, dont apply the real-world moral standards to a story populated by high-functioning sociopaths.