I agree that there should not be billionaires. It's immoral and unsustainable for any person (or family) to hoard that much wealth.
But killing them isn't a moral choice either, nor is it a viable path to economic justice. Their billions don't get redistributed to the masses. They go to their children, or to charities run by their friends, where a board of directors redistributes the funds amongst themselves.
We need economic reform, not violence and murder. We need a reasonable tax policy, citizen oversight into spending, and a functioning judicial system. The only way we win this war is at the polls.