It would be circular reasoning to believe capitalism is justified because only under capitalism can there be a select few people to know what to produce. If the society understands that it has interest in a certain product or service, then that society could create different ways (like under socialism) to put into place a person in charge of a worker controlled enterprise that would give general direction. This means that the enterprise would function as well as it would under capitalism minus the private profiteering and all other consequent ills that come from a capitalist mode of production.

Capital is self-expanding money. When you say you want to invest 'capital', what you want is your money to return to you greater than it was when you first put it in motion. You want to be able to pay, say, $100 worth of material and wages, then sell the assembled commodity for $150. This is how you make a profit. But where did this magical extra value come from? It came from the labourer you hired. You hire their labour power for a certain wage, after which they set about producing for you more than you paid them for. It has to be this way, otherwise you wouldn't make a profit. From the surplus of their labour you derive your profit. This is how your capital returns to you greater than before; this is how capital self-expands - by employing labour. You join the ranks of the capitalist class, and they join the ranks of the working class. They labour, and you appropriate the surplus they produce. Exploitation thus happens at the point of production through the interaction of capital, and wage labour.

An interesting thing to think about is why you are able to hire someone and exploit their labour like that in the first place. Why does the working class exist at all? Why did that employee ever work for you? It's because they have been deprived of the means to sustain their existence. They don't have farms, or factories, or offices -- private property belongs only to the capitalist class. Workers are forced to sell their labour power in order to acquire money with which they buy their food and shelter and other things necessary to reproduce their very existence. Thus you can see that capital and wage labour necessitate a certain form of violent property relations -- that a large group of people be denied the means to sustain their own lives and thus be forced to sell their life activity to a capitalist.