All things being equal taking from people that work hard and produce and giving to those that refuse to work is unjust. I think everyone recognizes that when one man goes into the field and works 12 hours per day cultivating a garden he is entitled to the fruits of his labor. Supposing his neighbor just doesn't want to do this kind of work and goofs off all summer. Winter comes and he has nothing. There's a basic sense of justness and fairness that says you made your bed, so lie in it. Assume the slacker slacked with his eyes open. He knew that if he didn't work he would be hungry come winter. Everybody understands this dynamic.
And we can take it a step further. Let's suppose the hard working man is elderly and has children. Let's suppose in the fall he dies before eating his food. Who gets this food? His children. Not the neighbor's children. We may take pity on them and give them food in the form of charity, but they are not entitled to the food. There's nothing wrong with the hard working man passing on this inheritance.
Now, if you are reading this as a conservative and you are thinking to yourself "Yeah, that's right, and this is why liberals are wrong" then there's something very important you need to recognize. You do not understand the disagreement between liberals and conservatives. You are not addressing the fundamental question.
It's just a basic fact that the ones that do the work on capitalism are not the ones that are paid the most. Perhaps you've heard of the 112 Bangladeshi garment workers burned up while sewing clothes together for Wal-Mart. They were making 18 cents/hour. They work extremely hard and produce a valuable product. They don't get a very large portion of the value they create. Who does? The Walton family heirs. People that not only aren't working in dangerous conditions, they aren't working at all.
Remember, the Walton family is not simply drawing from the monies left to them by their father. If that were true then eventually their money would run out and they would have to work like most people. They are being paid the largest share of the revenue that is today being created. Who is creating that value? Cashiers at the stores. Managers. Stock boys. Garment workers. The Walton family heirs probably do the least amount of work. Basically none. They get most of the money. Now, you can think that condition is for the best. You can make arguments for why it is just. I'm not addressing those arguments here. I'm simply saying that one argument for capitalism starts with the assumption that the hard working should be permitted to retain the value they create. Not only is this not denied on the left, it's quite the opposite of what capitalism produces.
The left doesn't have a problem with people being rich. So professional baseball players, movie stars, entrepreneurs. Sometimes CEO's and other managers. The problem is that capitalism clearly produces conditions where some of the people that get the most do the least amount of work. That is what the left is criticizing. Again, if you are OK with people like the Walton's who do the least amount of work getting most of the money that's fine. Just don't defend it saying that people that work hard should get to keep their money. That's exactly what is not happening in their case.