Well, I can’t speak for Communists or any other sort of statist as to the particulars of their resistance to Capitalism, but I can tell you my interpretation, as someone who leans heavily Mutualist, of anti-Capitalist reasoning.
Capital, in the case of factory-type capital, is not something that can be actually totally utilized by an individual, aka the Capitalist, that takes ownership of it. Instead, if they want to get full use out of its capacity, they pay a wage to workers to do it for them. The profits made thereby are the difference between the price the good is sold for and the costs of the labor, materials, and maintenance of the capital needed to produce it.
This can be seen in two ways. By those who support capitalism, it is seen as simply the Capitalist purchasing labor from workers. For the anti-Capitalist, this perspective ignores the true nature of the relationship. Rather than the Capitalist purchasing labor, it could be said that he is renting or loaning out the equipment for a portion of the product made therefrom. The Devil is in the details.
It is important to note, at this point, that the arguments raised by anti-capitalists apply mostly to factory-type production, the Capitalism of the industrial age. In many Western nations, we have moved (artificially and foolishly) away from actually producing anything of value, and the current situation is muddier. That being said, after some real pain from difficult changes, we will return to a more production-based (rather than merely finance- and service-based) economy, and these arguments will come back to the fore.
Continuing now with the infernal details, it turns out that, while Capitalists have never promised equality of outcome, they have mollified the masses with promises for equality of opportunity, or failing that at least some opportunity for everyone to improve their lot in life, and maybe even become wealthy. The truth of the situation is, alas, far less rosy. What has instead come about is the arising of a largely hereditary class system, seperated into the Capitalist Class and the Working Class, with definite and deliberate barriers in between.
The Capitalist Class tends to be better-educated, which helps cement their positions, but even so, the system is hardly meritocratic. It is simple fact that one who begins with wealth has a far easier time accumulating more wealth than one who begins with poverty. The members of the Capitalist Class, further, find it in many ways in their best interest to keep the wages they pay as close to subsistence level as possible. This has the immediate advantage of maximizing profits, but long-term it also prevents the worker from being able to accumulate enough to purchase their own equipment and become a competitor with their former employer.
Even this is not quite enough to condemn the system. If the great factories and division into classes are truly what result from a free market, AKA human interactions without coercion, it would be difficult to argue for altering this result. And indeed, this is just how many Anarcho-Capitalists, Objectivists, etc. see things coming about. It is this conception, claimed by many arch-Capitalists, which the anti-Capitalist looks at and therefore concludes that Capitalism is evil and ignorant.
This is because in a rational examination of the nature of industry and indeed ownership itself, we can see that Capitalism, in the sense of a system seperated into groups who make money by virtue of ownership and groups who make money by virtue of work, is unsustainable without the coercive arm of the state and enforced definitions of ownership derived therefrom.
It is easy enough to cover the difficulties the current system of centralized, massive production would face without socialized transportation (roads and rails built through eminent domain and funded through taxation), special trade deals with despots and tyrants for exclusive or semi-exclusive access to fuel and other raw materials, and massive government programs to absorb production overruns. Nor have massive corporations shown any ability to survive long-term without repeated bailouts from their governments, funded, of course, either through taxation or inflation.
This is because huge businesses are ponderous and inefficient things. The larger a business becomes, the more layers of bureaucracy (read: highly paid non-productive workers) are required, and its actions become less well-coordinated. Like a large state or empire, eventually a corporation grows so large that collapse is inevitable, its own bulk and clumsiness rendering all profits illusiory, a mere trick of bookeeping to satisfy blissfully ignorant shareholders. Of course, without the interference of the government, a business would fall to smaller, more efficient competitors long before it collapsed on its own.
In short, small efficient, localized production, largely by workers who were also owners of their small-scale capital, would outperform and outlast larger, bulkier competitors without the controls and aids put in place to encourage larger organizations to form. There would many different businesses, such that they would have to compete for what workers were available far more than the giant, centralized corporations of today do, and thus pay them far closer to the actual value of their production. Indeed, this process would eventually result in the end of profit, as any profits taken would make one less able to compete for workers. In the end, any business with employees would have to pay managers and workers the full value of their work, with only the costs of maintenance and materials excluded, and so would have no profits at all. The value of being a mere owner would disappear.
Even insofar as larger scales of production persisted, organized labor would end the profit system. While laws today force union recognition on companies, they also tie the hands of organized labor, converting it into a de facto apparatus of the Capitalist profit system. Laws force union recognition, but ban general strikes, disallow anyone labeled “manager” from joining, and prevent people in particularly sensitive industries from striking at all. In the office I work in, there are two “managers” who manage absolutely no one, and one “Director” (read: grand high muckety-muck) with a similar level of responsibility. It’s a joke.
The general strike was once a devastating tool of organized labor that could bring the Capitalist profit system to its knees. One factory or one part of an industry going on strike can be compensated for with scabs, but when every step of production, not just the one factory but their supplier and their distributor, all go on strike at the same time, there is just no way to compensate. The Capitalist either has to sacrifice all or part of his profits to get production going, or close down, leaving the capital unused, and will probably have to sell it at a considerable loss. And the best part is, for such a strike to work requires no force at all, but only coordination and determination.
To sum up, the Capitalist system is seen as evil because inherent to it, according to its proponents, are aspects sustainable only through controlled markets, AKA force. I have tried to demonstrate here that profits via wage-earning labor is impossible without force and injustice. I imagine most of you will think I failed, if you have even bothered to read through this slightly scattered and quite long-winded rant o’mine, but I wanted to put it out there.