If we suppose that the goal of society is to produce the greatest utility, and that the utility wealth provides an individual is sub-linear (i.e. twice as much money makes you less than twice as happy), then inequality is inefficient resource allocation.

However, we also suppose that some level of inequality can lead to greater productivity, and thus greater utility overall. The question is then what level produces the best outcome?

Personally, I don't think the current levels of inequality are remotely optimal. We could drastically improve thousands of people's lives for amounts of money that some rich individuals wouldn't get out of bed for, so I'm not convinced that reallocating that wealth would be harmful overall.