Not 70% of someone's income, just 70% of only the portion of any single person's income over $10,000,000 per year. Morally justified the same way all taxes are justified.
The old question is whether this results in less motivation to do work which brings you above the (whatever) top tax bracket. In my experience it actually is.
Also, I do understand your point of view. When I was an ardent Ayn Rand fan (much younger then), I used to use this same argument, an in fact the same phrasing: taking income at the threat of violence (which, IIRC, is from Capitalism: The Unknown Ideal). I've just come to realize over the years that the opposing viewpoint is actually morally superior.
I see no issues with some having 1000x the wealth of others. We both probably have 1000x the average citizen in some of the poorest countries in the world. Is this a problem yes. Are we morally wrong for having that wealth, no I don't think so.
Is there a problem with the difference in power at the political level, yes, is this a moral issue yes. But I don't think taking from the rich giving to the poor solves this problem. The reason being that as we do this it tends to make the situation worse. Welfare programs tend to restrict the freedom of those on the programs more that the rest of the population and have been shown to develop a learned dependence on these programs. Even if it did work I don't see how that justifies taking so much from someone.
It's a good idea to try and level the playing field. But I don't think massive income taxes are moral and as such should not be chosen as the tool to do this.
You don't have to agree to the laws of a democratic constitutional republic, but you sure do have to follow them.
And, really? Theft is never morally justified? Never ever?
and theft is never morally justified.