Oh no the brother trope again... And the brother is a bitch (￣∇￣") well I'll just wa...

While granted they were teens arguing in the way teens will, the older brother hasn't been shown in very favorable light. Whether he was just saying stuff to be provoking, because he too felt trapped by their family situation or not, what he said and did was very insensitive and hard to justify given the circumstances. Meaning, while he might normally say provoking things to his brother (being a teen and a brother) there are times you just don't because the subject is just too serious or hurtful. Gaon was having to face the full reality of what being a vampire would mean, so it was either very insensitive or very cruel of him to act the way he did.