People on the left will often justify communism by pointing out abuses that have happened under capitalist system. Yes, there have been abuses under capitalist systems (e.g. the mass killings in Belgian Congo at rubber plantations in the late nineteenth century), but the problem with the left's assessment is that such abuses are far and few between. There have been few communist regimes in history, but mass slaughter has occurred in a high number of them. In contrast, there have been a vast number of capitalist systems, but mass slaughter has occurred in very few of them.
Another problem is that capitalism itself is not a political system; it is an economic one. Communism, in contrast, is an economic system that presupposes a certain (authoritarian) political system in order to exist. When leftists push communism as the "answer", they are not just pushing against capitalism, but also against democratic government (democracy). In other words, by opposing democracy, leftists are opposing indivudal freedom.
This is why leftism is so dangerous; its adherents want to strip away freedoms from people so that they can achieve their (naive) utopian vision. We can stop and analyze this utopian vision of theirs that they think so highly of, but perhaps that's for another time in another thread...
