Killing other humans is often justified. For instance, it would be completely justified to kill someone who was in the process of shooting up a school.
In the same way, billionaires are guilty of causing the deaths of millions of people through their hoarding of necessities. Killing the billionaires would allow this wealth to be redistributed, saving potentially millions of lives.
It absolutely is a violent act. In the same way that locking a person in a cage is a violent act, depriving people of the things they need to live is a violent act.
The fact is, simply asking these people to stop hoarding and polluting is not going to work (duh).
Is there a non-violent solution?