In general terms, communism is flawed and therefore deemed "bad/wrong" because power is centralized, typically, in the hands of one person with a great chain of command. Based on my readings, I know Stalin and Tse-Tung ruthlessly killed their underlings in their paranoia. From my reading of Mao, I know he contributed to the murder of his second wife. Millions died of famine due to his inaction..
But for communism itself, I'll quote my PDF version of the Communist Manifesto later (I can't copy/paste or patch in links, etc). I do know vast power is given to the government in order to make communism work. I know people don't get to own stuff; they merely work to earn their food. Iirc, there's... 10 planks or precepts? I can't recall. Simply put communism seems to drain the individual's ability to own property, run a business, compete/innovate, etc..