Quoting from the article you are referring to:
> He recommends a global wealth tax and, for the United States, top income tax rates of 80 percent on income above $500,000 or $1 million to combat inequality and 50 or 60 percent on income above about $200,000 to combat inequality and grow the government.
First of all I haven't read the book yet so can't comment on the numbers.This limit can very well be a million or a few millions. He is arguing is that effective tax rate on multinational companies is lower than small and medium scale businesses. This isn't good for capitalism. In fact this will just hasten the process towards concentration of resources in the hands of few organization or groups. Will it look like a capitalist society anymore when a few big groups own everything and others compete for minimum wages?
> The violence of Law should be applied exclusively against those who violate the human rights of others. It should not be used to take property from one group to give to another.
You are circumventing the basic question. From where property came from? Did individual said "Let there be property" and property came into existence?
No, property is a result of individual's or an organized group's utilization of natural or human resources. Piketty is not arguing to take it away with force in communist fashion. All he is arguing is that tax should increase with income which is contrary to what is happening on the ground nowadays[1][2].
> I grab some clay, and create a nice figurine. You give me some sea shells in exchange for it. No one has a right to a share of my sea shells.
We are discussing taxation of higher income not some petty transaction between individual player. However, if we take your argument to its logical conclusion and imagine a billion dollar Multi-National making the same argument that I am just bartering few thousand tonnes of clay with few hundred tonnes of seashell so we aren't responsible for soil erosion and extinction of species.
Only reason not to tax large groups was that they generate employment and moves the society forward by investing money into research. This is not true research [3] as well as jobs[4].
[1] http://www.cnbc.com/2015/03/10/why-we-need-a-global-wealth-t...
[2] http://money.cnn.com/2013/03/04/news/economy/buffett-secreta...
[3] https://www.ted.com/talks/mariana_mazzucato_government_inves...
[4] http://www.truthfulpolitics.com/http:/truthfulpolitics.com/c...