According to Karl Marx, the evil of capitalism is not the tension it fosters or the supposed lack of concern for the poor. But the problem of overwork is inevitable. For Marx, capitalism was a flawed system because it inevitably deprived the worker of some of the fruits of his labor. While his theory itself is fundamentally flawed, it is nevertheless the basis of much communist doctrine.
The theory of surplus value/labor is based on the assumption that the cost of a product is determined by how much labor is required to make it. But according to what sounds logical, capitalism is taken from another assumption. Capitalists believe that goods are worth whatever someone is willing to pay for them. This seems to match what we see a little better in the world. Consider this example. A farmer is cultivating a field when he arrives with a huge diamond plow buried in the ground. He takes it, takes it into town, and sells it to a jeweler for several thousand dollars. Then he returns and works until the harvest. When he has gathered his crop, he takes it into town and sells it at the market. According to the communist theory of value, its fruit should be much more valuable than diamonds, because much more labor is required to pick them than diamonds. Of course, we know that this is not what we see in the real world.
However, Marx built on this assumption to develop his theory of surplus labor. Because Marx believed that goods were only as valuable as the amount of labor, he thought that any additional costs from the wealthy capitalists, who invested in the labor of the workers, would drive the price down. To his mind, if a worker’s labor produces 20 worth of goods in one hour, he should be paid $20 per hour. If less, how much was the value of what he had added, how much profit would he be robbed of? Of course, businesses have other expenses besides labor, but the main question, as Marx saw it, was what to do with the profits. Marx believed that there was no extra profit when the worker took all the net profit he produced.
What Marx could not answer was how factories were to be built, how management was to be organized, and how capital was to be raised without creating surplus value . These things must be done for the workers, so that they can do their job. It is not the exploitation of capitalists, but the necessity of current in modern business. Some products are left for marketing, distribution, research, and management. Factory owners receive a portion of what the workers make in order to keep the factory open. They also have to eat. Marx saw all that as a form of theft, which is why he wanted the state to control the means of production.
References
International Viewpoint
Marxists.org
Mount Holyoke College