Capitalism is flawed and has outlived it's usefulness just as every preceding economic system has. One of the more poignant Marx quotes puts it well

The history of all hitherto existing society is the history of class struggles. Freeman and slave, patrician and plebeian, lord and serf, guildmaster and journeyman, in a word, oppressor and oppressed, stood in constant opposition to one another, carried on an uninterrupted, now hidden, now open fight, that each time ended, either in the revolutionary reconstitution of society at large, or in the common ruin of the contending classes.

Capitalism is based on the accumulation of resources known as the "means of production". As time goes on, those with capital are able to leverage it to further subjugate the working class as they amass a disproportionate amount of wealth and capital. The average worker is worth far more than they are paid, while the capitalist who they work under continues to pocket the majority of that profit.

For a working class person to begin to earn their fare share they have a few ethical options, be self employed, unionize to collectively bargain for a larger piece of the pie, join or form a co-op (effectively a small scale form of socialism).

The last point I'd bring up that is more central to my own politics is the inherent link between capitalism and imperialism. Even in a capitalist country where you may be able to comfortably live as a member of the working class, the global third world is often footing the bill in order to lower the cost of goods. Examples would be clothing, chocolate, coffee, etc where most of these are made in desolate conditions and sometimes with slave labor.

That being said, there are many reasons to be against capitalism and it is hard to express in a single comment. I highly recommend Lenin's State and Revolution to anyone interested.