Under the flat tax plan, lets say you have a 40% tax rate. Someone who makes One-million a year pays 400K and they still have plenty to spend on any other activities. Someone who makes 20k a year (Just below the poverty line) pays 8K, leaving that person far worse off than they were before, While the wealthy individual can literally buy a house every year (If s/he so desires).
Taxes should slowly increase percentage wise for every single extra dollar you make. Every Dollar means far more to a poorer individual that a wealthier individual. We should not keep people poor, but allow them to have a basic standard of living, so that they don't suffer economically. A given society is pointless if it can't do anything for the greater good of the people.
It's not the "Poor punishing the rich", because the same gradual tax increases goes for everyone. If a poor guy eventually becomes wealthy off of the system/society, It is reasonable that he gives back into the system that helped him increase his profit margins. If a rich guy becomes poor (or poorer), It makes sense to help him maintain a basic standard of living so he does not suffer economically (So decrease his taxes).
It's fair that people who make more money should have to pay higher taxes. They have more money to spend, so they can tolerate paying more. Also, they have benefitted more from living in this country, and they need to pay the government back. It's also unfair to tax the very poor the same as the very rich.
I believe everyone should be taxed at the same rate, rich and poor young and old. Its a misconception that the poor are disproportionately affected by a flat tax. Wages are higher with a flat tax rate. The United States would be fine with a 12 percent flat tax rate.
The rich will pay more in tax anyway. And by taxing the rich at a higher rate you actually stop income mobility. You make it harder for people to go up the income ladder. This happens because people find work is actually paying less the more they work. It is an evil incentive. And if taking 40% of someone making $12,000 a year is too much then maybe there is also a problem of how high taxes are.