There's an argument to be made that the 10% made their money off the backs of the 90%, but I won't go into that. I also don't agree with the inheritance/death taxes, personally.
While something might not be justified "morally", per se, we run into issues where what's best for the nation isn't justifiable on an individual basis. For instance, education taxes. If I don't have a kid in school, is it morally justifiable for the gov't to require money from me?
But long term, said child's education will be supporting our nation's infrastructure/future.
If an extreme gap in wealth equality leads to an increased likelihood of mass riots/protests/etc etc, shouldn't we look for ways to avoid those outcomes?
Sometimes the "optimal" way to do things isn't really optimal. (See Game Theory and actual outcomes.)