The capitalist rise has been associated with exploitation, mass executions, imperialist wars, banana republics, extension of international slavery, human trafficking, and inhumane living and working conditions for workers (not only for slaves, mind you).
Lots of unchecked power and lack of historical precendents have a tendency to build flawed societies, that's no secret.
But, just as with the history of capitalism, now we can learn from the mistakes made and correct the course. We have positive examples (early USSR driven internally by worker's councils, i.e., well, Soviets; Chile with democratically elected socialist leadership; Yugoslavia with League of Communists being elected by the people and republical committees; et cetera et cetera), and we have negative ones (Stalin's USSR, Mao's China etc.). We know what to do, and we are not doomed to repeat this in whole, just as we don't have to come back to slavery despite it being present in capitalist societies before.
On the economic side, a lot of it stems from the original disparity in the amount of capital, so just tracing who got more at the end isn't fair; what is fair is to compare economic dynamics, and on that part socialism wins hands down, especially in times of an industrial revolution. Socialism allows for a rapid change, it is the system that produced some of the highest GDP growths ever recorded, it is the system that took people out of the poverty loop and dustributed the new gains fairly.
But what we're left with if we continue our course under capitalism? Continued concentration of wealth and unchecked power of certain people over our economy. Those people were not elected, they are not responsible to us, they can do what they please with the capital they have accumulated. And they always get more and more, not just in absolute numbers, but percentage-wise too. This goes in full accordance with an economic theory, and this trend will not change any time in the future.
So essentially we're left to three choices:
-Continue with status quo and end up under the control of unelected people with unchecked power (see the first paragraph)
-"Starting again", forcefully redistributing wealth back to an equal state, only to come to this point again later on
-Building a functioning socialist society and finally moving on to the new chapter in the development of humanity.
What do you pick?