If you really want a deep dive into this topic...
My sense of morality is based on pragmatism in pursuit of utilitarianism. The goal should be the greatest good for the greatest number of people, and the strategy to pursue that goal should be whatever works. 100% income tax wouldn't work. That's basically communism, and it's been tried, and it's failed spectacularly. People need to be able to keep some of the fruits of their labor so they have some incentive to actually produce things, or nothing gets done. Trickle down economics has also been tried before, and while it works wonderfully for a small class of super rich owners of capital, it doesn't work at all for most of the population, thus failing the "for the greatest number of people" part of utilitarianism. What does work is progressive taxation used to fund progressive wealth distribution, with very high (but not 100%) taxes for the very rich, sort of like exactly what Clinton is proposing.
Inheritance tax is not exactly the same as income tax, but it has some similarities. From a libertarian, rather than utilitarian, moral worldview, it seems like you have much more of a moral claim to wealth you've earned yourself rather than wealth you inherit, thereby making inheritance tax at least somewhat more morally justifiable than income tax. But, you could argue that you have a libertarian moral right to decide where your wealth will go after you die (this seems iffy to me... what difference does it make to you if you're already dead?) which could raise a libertarian moral objection to inheritance tax. From a pragmatic utilitarian moral worldview, a 65% inheritance tax on billion-dollar estates is a complete no-brainer. Yes, you do want to create incentives to be productive, and the possibility of leaving a large inheritance to your kids is a big incentive. But leaving 35% of any additional money you make beyond a billion dollars in plenty incentive enough, and the harm done by removing the incentive to make more money is far outweighed by the good done by reducing generational class stratification.
As for why I haven't yet given away all my money in pursuit of utilitarianism... well, I don't act 100% altruistically all the time. Guilty as charged. That isn't going to stop me from considering the good of others besides myself when deciding who I vote for.
