Yeah, but those things have a different outcome depending on whether it's a single individual or a society of individuals doing them (in that example: police being accountable to one person instead of the public as a whole).

This is different, since no matter what, an old man voluntarily dies for the financial benefit of his descendants. If the outcome is the same no matter who makes that happen, then I fail to see why one approach to doing so would be more or less moral than the other.

If anything, a single individual purchasing that old man's life would be more moral, since the alternative would be to compel an entire society (and specifically the members thereof) to bear that cost (both monetarily - i.e. via taxes - and the emotional cost of having killed someone). Given that the billionaire is (presumably) a member of society, the net impact is identical, but it's compartmentalized to a single individual who volunteered for those costs versus an entire society of individuals who might not have.