I know it's extreme. And it's admittedly an exaggeration, but I stand by the logic. The original post, while maybe not the most traditional venue for a topic as serious a death, showed real sincerity. (We have to assume this guy's brother really died, or there is no discussion here.) The flippant reply failed to acknowledge that and did two things:

First, wow gold lightened the mood when no mood lightening was called for. It would be equivalent to someone jumping up and ripping a huge fart right after someone's teary obituary. What might be funny in one situation suddenly becomes horrifying in another. The fact that this was a complete stranger makes it even worse. Members of the family or a close friends might be able to joyfully reminisce at someone's departing, but who would appreciate a stranger off the street suddenly coming in and making jokes at their dearly departed's expense?